Mickey summons the Doctor and Rose to Deffry Vale High School, which he believes is the site of an alien incursion. Since the arrival of a sinister new headmaster, Lucas Finch, academic performance is soaring -- but some of the students have started to go missing. The Doctor poses as a teacher, while Rose becomes a dinner lady and discovers that there is something peculiar about a special cooking oil. Someone else has also grown suspicious of Deffry Vale: a journalist by the name of Sarah Jane Smith. Reunited with the Doctor, she must help him uncover the plans of the extraterrestrial scavengers known as the Krillitanes.

On December 8th, 2003, executive producer Russell T Davies circulated a pitch document for his revival of Doctor Who. One element of this outline which did not survive into production was the inclusion of the Doctor's robot dog, K·9. Introduced in 1977's The Invisible Enemy, K·9 had proved extremely popular with young viewers. When then-producer John Nathan-Turner decided to remove him from Doctor Who at the conclusion of Warriors' Gate in January 1981, the metal mutt's enduring appeal prompted the creation of a spin-off show entitled K·9 And Company. In the event, however, only pilot episode A Girl's Best Friend went before the cameras, to be broadcast the following Christmas. Nonetheless, K·9 was back again two years later, making a cameo appearance in the twentieth-anniversary special, The Five Doctors, and then the thirtieth-anniversary tribute, Dimensions In Time; he subsequently maintained a persistent presence in various Doctor Who spin-off media.

Apart from K·9, Davies was also interested in revisiting one of the Doctor's former human companions. For the most part disinclined to dwell on the programme's lengthy history, Davies nonetheless saw this idea as a way to shed new light on the fate of those who travel with the Doctor, with attendant implications for the current companion, Rose Tyler. Davies initially thought that the concept might be feasible should the Doctor Who relaunch survive into a third season. By early 2005, however, he had had a change of heart and intended to position the story late in its sophomore year.

Sarah Jane Smith was often cited as a quintessential Doctor Who character

The returning character whom Davies wanted to spotlight was Sarah Jane Smith. Arguably Doctor Who's most iconic companion, the investigative journalist had been played by Elisabeth Sladen from 1973's The Time Warrior to 1976's The Hand Of Fear, spanning the regeneration from Jon Pertwee's Third Doctor to Tom Baker's Fourth. Sarah had been associated with K·9 in both A Girl's Best Friend and The Five Doctors, and she was often cited as a quintessential Doctor Who character, remembered even by those who had been only casual viewers of the programme. Indeed, like K·9, Sarah Jane made frequent appearances in Doctor Who tie-ins, with Sladen reprising the role in everything from Dimensions In Time to the video drama Downtime to a series of audio dramas in which she was reunited with Pertwee's Doctor.

In early 2005, the Doctor Who production office approached Sladen about returning to the series. She was immediately apprehensive, fearing that she was being invited back for a throwaway cameo. However, after a dinner meeting with Davies and producer Phil Collinson, Sladen was delighted to learn that they intended Sarah to be the focal point of an adventure. She also found that their vision of the character coincided with her own thoughts about how Sarah Jane would have matured after leaving the Doctor.

At this stage, the Sarah Jane story was entitled “Old Friends”, and was intended to air late in Season Twenty-Eight. Davies assigned the scripting duties to Toby Whithouse, who was given no parameters beyond the involvement of both Sarah and K·9. Whithouse's initial idea concerned an alien race which sought revenge on the Doctor for his actions in the distant past. However, the production team indicated that they favoured storylines in which the Doctor involved himself in events, rather than instigating them.

Instead, Davies proposed a narrative set around a rural village and a nearby army base. Whithouse devised a treatment entitled “Black Ops” in which Sarah investigated alien activity at the base, as a result of which the townsfolk were developing advanced scientific knowledge and engineering a sophisticated bomb. Whithouse initially omitted K·9 from the storyline because he felt that the character would be out of place in Doctor Who's modern incarnation. However, he soon found himself reinserting the robot dog at the insistence of the production team. A set piece conceived by Davies saw Sarah Jane kidnapped by the aliens, with the Doctor piloting the TARDIS along a motorway in pursuit of their car.

As a boy, Russell T Davies had imagined the Doctor showing up in class disguised as a teacher

“Black Ops” was greeted with enthusiasm, but Davies soon deemed the army base setting a mistake, as it was forcing Whithouse to jump through too many storytelling hoops. Instead, he proposed transplanting the action to a comprehensive school; as a boy, he had imagined the Doctor showing up in class disguised as a teacher, and he had used the setting himself in the 1991 family thriller Dark Season. Despite this, it was originally Sarah, rather than the Doctor, who went undercover as a teacher. The new locale suggested the inclusion of the cooking oil subplot, drawing upon the recent controversy over school menus which had been incited by chef Jamie Oliver's 2005 documentary series Jamie's School Dinners. The car chase no longer suited the revised narrative, and was dropped. Davies repurposed it for The Runaway Bride, which at that stage was intended to air later in the 2006 season, although it was eventually reimagined as the year's Christmas special.

Whithouse began writing his first draft script around early April 2005. It was at about this time that the adventure was brought forward to the third position in the season's running order. As such, the writer was asked to incorporate Mickey Smith into the plot, with the character ultimately joining the TARDIS crew. The appearance of K·9 Mark IV at the episode's conclusion was omitted from early drafts. Its inclusion was one of the few requests made by K·9's co-creator, Bob Baker, although Davies had already asked Whithouse to add such a scene.

Since “Black Ops” was no longer a suitable title for the adventure, Whithouse renamed it “Friends Reunited”. This was a reference to the same-named social networking website, which had been one of the first to achieve prominence in the United Kingdom. Executive producer Julie Gardner instead offered School Reunion, which Whithouse came to prefer. The headmaster was called Hector Finch until it was learned that there was a real teacher by that name; he was rechristened Lucas Finch. The aliens were initially Krillians but, when it was discovered that the term was already trademarked by a computer company, they became Krillitanes. The aliens' physical appearance also evolved during the story's development, with Whithouse first envisaging them as more bat-like, with blue fur. Later, he hoped that the otherwise human Finch could be seen to fly along the school corridors on membranous wings, but the budget could not accommodate such an amalgamation of live action and computer animation. The eventual depiction of the Krillitanes in their alien forms was inspired by stop-motion animator Ray Harryhausen's rendition of the mythical Harpies in the 1963 movie Jason And The Argonauts.

There was little time to spare before the teaching staff resumed work at the two schools representing Deffry Vale High

School Reunion was originally planned to be part of Season Twenty-Eight's second production block. However, it was soon realised that it would need to be brought forward into Block One in order to facilitate recording on school grounds during the summer holidays. In turn, Tooth And Claw was delayed to Block Two, providing time to resolve its scripting issues. This meant that School Reunion would be directed by James Hawes alongside The Christmas Invasion and New Earth. The festive special was Hawes' initial priority, and problems during its production meant that work on School Reunion started later than anticipated. This left the cast and crew with little time to spare before the teaching staff resumed work at the two schools chosen to represent Deffry Vale High.

The first of these was Fitzalan High School in Cardiff, where filming took place on August 23rd and 24th. After visiting the location, Hawes made several adjustments to the scripted action: the TARDIS was originally hidden in a supply closet rather than the gymnasium, while the Doctor's confrontation with Finch was moved from the gym to the pool. Cameras rolled in the gym on both days; the 23rd also saw the filming of scenes set in the kitchen yard, the playground and the pool, followed by those in the cafeteria and the kitchen on the 24th. On the second day, Sladen suffered a tendon injury after slipping on the gym floor. As a result, third assistant director Lynsey Muir -- bedecked in an appropriate wig -- stood in as Sarah Jane whenever it was feasible.

The new K·9 prop debuted on August 24th, having been constructed by Mike Tucker's Model Unit team with reference to an authentic Seventies K·9 prop he owned. Unfortunately, the robot dog's tradition of struggling with movement over various surfaces continued into the new century, the tiled kitchen floor posing a particular challenge. Furthermore, during the climactic kitchen sequence, K·9's eyepiece was made to flash in time with his dialogue. This was out of keeping with the manner in which the robot dog had been portrayed in the past and, when Collinson learned of it upon his return from a holiday, he indicated that the practice should be abandoned for the remainder of K·9's scenes.

The major location for School Reunion was Duffryn High School in Newport, where all of the remaining Deffry Vale scenes were recorded. Shooting there began on August 25th and was planned to continue until September 2nd, omitting only August 28th. The extras playing Deffry Vale students were genuine Duffryn schoolchildren, who were bussed to both Fitzalan and their alma mater to participate in the filming. Unfortunately, the production block continued to fall behind schedule during the School Reunion shoot, and it quickly became clear that an additional day at Duffryn High would be needed. The situation was exacerbated on September 2nd when work on the scene of Mickey crashing Sarah's car into the front of the school -- actually a false porch constructed by the BBC -- was held up by the discovery of asbestos in Duffryn High's roof. As a result, an extra day of filming at the location was arranged for September 6th. Some sequences were abandoned or simplified, notably a time-lapse shot of the school corridors to bridge the daytime and nighttime material.

Prominent in the schedule for September 7th was the concluding material in which Sarah Jane entered the TARDIS. It had been written to take place in the ruins of Deffry Vale, and Hawes' team had negotiated the use of a partly dismantled British Telecom building for the purpose. At short notice, however, it was learned that the structure had now been completely demolished, and so the shots were instead filmed in Newport's Belle View Park, with the unscripted implication that the Doctor had repositioned the TARDIS. The K·9 prop used in this scene was a refurbished version of the original radio-controlled unit built for The Invisible Enemy, now in the possession of former Doctor Who visual effects designer Mat Irvine.

The rest of September 7th was spent at Da Vinci's Coffee Shop in Newport for scenes in the cafe. On the 8th, the TARDIS sequence was filmed at Unit Q2, Doctor Who's regular Newport studio space. Later that day, cast and crew returned to Da Vinci's, this time to shoot cafe exteriors; unfortunately, their work was hampered by the presence of several unruly drunks. September 8th was also the day that model footage of the Deffry Vale explosion was recorded at the BBC Model Unit Stage in Acton, London. This was the final filming ever undertaken by the BBC's Visual Effects Department, which was subsequently disbanded.

On February 2nd, John Leeson returned to the role he had originated in 1977, when he recorded all of K·9's dialogue

The last scene to be captured for School Reunion was Mickey in the cybercafe, taped at Unit Q2 on October 8th. A similar setting was revisited when Noel Clarke returned to film the corresponding TARDISode on January 31st, 2006 at Enfys Television Studios in Cardiff. The fifty-second prelude was written by Gareth Roberts, and saw Mickey phoning Rose to alert her to the strange goings-on at Deffry Vale. It also furthered the mystery of the enigmatic Torchwood organisation, whose existence was first suggested in 2005's Bad Wolf and then more fully established in Tooth And Claw. Two days later, on February 2nd, John Leeson returned to the role he had originated in 1977, when he recorded all of K·9's dialogue at AIR Studios in Hampstead, London.

The TARDISode was released on April 22nd, while School Reunion itself was broadcast on the 29th. As with the season premiere, New Earth, two weeks earlier, Doctor Who was positioned in the BBC One schedule between Strictly Dance Fever and The National Lottery: Jet Set. However, its timeslot was now five minutes later, at 7.20pm.

Meanwhile, the return of Sarah Jane Smith had been deemed a resounding success, both amongst the Doctor Who production team and farther up the BBC hierarchy. Davies began to consider bringing Sladen back to the programme on an annual basis, but discussions soon escalated to a fully-fledged spin-off series based around her character. Finally, on September 14th, it was announced that The Sarah Jane Adventures would debut in early 2007 with Sladen once again returning to the role. Sarah's travels with the Doctor may have ended, but much of her story remained to be told...

• Doctor Who Magazine Special Edition #14, 9th November 2006, “Episode 3: School Reunion” by Andrew Pixley, Panini Publishing Ltd.
• Doctor Who: The Complete History #52, 2016, “Story 170: School Reunion”, edited by John Ainsworth, Hachette Partworks Ltd.
• Doctor Who: The Inside Story by Gary Russell (2006), BBC Books.