"School Reunion" is the third episode in the second series of the British science fiction television series Doctor Who. It first aired on 29 April 2006. The episode's narrative takes place some time after the events of "The Christmas Invasion". In the episode's TARDISODE, Mickey Smith alerts the Doctor and Rose Tyler to a comprehensive school where strange things are happening, such as UFO sightings. At the school, the Doctor encounters his former companion Sarah Jane Smith and the robotic dog, K9 Mark III, he sent her, who likewise were interested by the current events. Together, they discover the force behind the events, a race known as the Krillitanes, and work to prevent them gaining control of the universe.
The use of the Doctor's previous companions, in particular Sarah Jane and K9, was first proposed in 2003 to the BBC. After the episode was produced, Elisabeth Sladen was approached by the BBC to star in a spin-off, The Sarah Jane Adventures, which also included K9 in several stories. The episode was positively reviewed, with an Appreciation Index of 85% ("Excellent").
Mr. Finch, the headmaster of Deffry Vale School, has been changing the school to improve the students' performance; his changes include free lunches with special chips. The Doctor, under the alias "John Smith", is undercover as a science teacher in the school, and his companion, Rose Tyler, is working undercover in the school's cafeteria. The Doctor is surprised by the good behavior of the students and intrigued by the uncommon intelligence of one of his physics students. Both the Doctor and Rose relate the mysterious events around the school to the chips: Rose observes that the chip oil has an adverse effect on the other kitchen staff, who must use hazmat suits to handle the oil, while the Doctor notes that the chips themselves are making children more intelligent.
Mr. Finch's successes have aroused media attention — in particular, the attention of investigative journalist Sarah Jane Smith. While at first the Doctor feigns ignorance (but is barely able to restrain his pleasure at seeing her again), when Sarah Jane discovers the TARDIS during the night, he reveals his identity to her. Sarah Jane meets Rose and Mickey, with an immediate rivalry sparking between the two women. As the group searches the school, they find thirteen bat-like creatures asleep in Finch's office. Returning to Sarah Jane's car, the journalist reveals a battered, inactive K9 Mark III, who—once repaired—identifies the chip oil as Krillitane oil. Rose, unaware that the Doctor had even had past companions, confronts him; he replies sadly that, while she can spend the rest of her life with him, he cannot spend the rest of his life with her, so he must remain aloof.
The following day, the group returns to the school to investigate further. The Doctor confronts Mr. Finch, who confirms that he and the others are Krillitanes — a composite species that takes desirable attributes of the species they conquer. He attempts to subvert the Doctor, without success. Meanwhile, Sarah Jane and Rose, now getting on better than at their first meeting, try to unlock the programming on the school's computers; Mickey and K9 remain in Sarah Jane's car for surveillance. With the Doctor's help, they discover that the computers — bolstered by the students' enhanced intelligence — are part of a Krillitane effort to solve the "Skasis Paradigm", a theory of everything. As the paradigm is nearly solved, which would allow the Krillitanes full control over time and space, they seal the school. Mr. Finch propositions the Doctor a second time, tempting him with the ability to save the Time Lords and override human mortality. With Sarah Jane's counseling, the Doctor refuses, and they run from Mr. Finch. After Kenny (a student who had not eaten the chips) alerts Mickey to the students' plight, Mickey crashes Sarah Jane's car through the school's doors and unplugs the computers, allowing the children to flee. The Doctor leads the Krillitanes to the kitchen. Upon their arrival, K9 detonates the chip oil container, saturating the Krillitanes. The Krillitanes suddenly explode, destroying the school and K9.
At episode's end, Rose, Sarah Jane, Mickey, and the Doctor are standing in the TARDIS. Sarah Jane declines a second chance of travelling in the TARDIS, having finally decided to move on with her life and stop waiting for the Doctor. Mickey says he would like to join the TARDIS crew, a suggestion of which Sarah Jane approves. She then asks Rose to stay with the Doctor, and to find her one-day if she needs to. Sarah Jane then gets a chance to part properly with the Doctor, who leaves her with a brand new K9 as a parting gift.
"School Reunion" is the first appearance of Sarah Jane Smith and K9 since The Five Doctors. It answers the question of where Sarah was left at the end of The Hand of Fear: Aberdeen. Rose mentions visiting "the year 5 billion", a reference to her first trip with the Ninth Doctor in The End of the World. The bragging match between Sarah and Rose also references other stories: Sarah has met the Daleks, "lots of robots", anti-matter monsters, mummies, dinosaurs, and the Loch Ness Monster, whereas Rose has met ghosts, "Slitheen in Downing Street", the Dalek Emperor, gas-masked zombies, and a werewolf.
The concept of Sarah-Jane and K9 returning to Doctor Who was an idea of Russell T Davies from pitching the show in 2003. Such a use would show what would happen after a companion left the Doctor, without dwelling too much on the classic series. It was Davies' full intention for Sarah Jane to be used for this, and while Sladen originally declined a request, thinking her role would not be important, she changed her mind when she realised she would be the focal point of the adventure. After production of the episode was finished, Sladen was approached about a full spin-off series, The Sarah Jane Adventures, which was formally announced on 14 September 2006.
The episode went through several changes in production: working titles included "Old Friends" and "Black Ops", the latter being set in an army base. Davies requested that Whithouse set it in a school instead, mainly for simplicity, but also for a desire for the Doctor to masquerade as a school teacher. Additionally, the Krillitanes were to be named "Krillians" until the BBC found the name was trademarked, and Finch's forename was originally Hector, until the BBC found a real teacher by the same name, and renamed him Lucas. K9's eye grill was initially filmed flashing in sync with his lines, not unlike the Daleks, until Phil Collinson saw the footage and nixed the practice (which was inconsistent with the robot dog's past appearances). A scene that was cut was of Milo's brain being "shorted out" by the Doctor's rapid-fire questions at the beginning of the episode, which was later alluded to in the episode.
The episode, originally in the second production block, was produced in the first block along with "The Christmas Invasion" and "New Earth". Two high schools in Wales were used for filming: Fitzalan High School in Leckwith was used on 23 August and 24 August 2005, for filming the first conversation between the Doctor and Finch, and for the playground, kitchen, and cafeteria scenes, and Duffryn High School in Newport, which was used between 25 August and 6 September for the remainder of the episode, with filming delayed due to asbestos being discovered in Duffryn High School's structure. The scenes in the schools utilised dozens of children as extras. Pick-up shots were later completed on 7 September and 8 September, with filming of the cafe scene delayed due to drunk and disorderly conduct from members of the public.
The episode was watched by 8.3 million viewers, the twelfth most-watched programme of the week, with an Appreciation Index score of 85%. Jacob Clifton of Television Without Pity gave the episode an A+ rating, and jokingly stated that he "didn't know why the Doctor was fucking around [during the Physics lesson]: he taught Physics at Coal Hill School way back in '63". Ahsan Haque of IGN gave the episode an 8.7 out of 10 ("Great") and commented that the episode had "fantastic character moments" and "brilliant CGI effects", and that "if you're willing to accept the Scooby-Doo storyline, then the strong nostalgic vibes present in this episode should be enough to carry this episode into a must-see category.", and K9 and Sarah-Jane alone made the episode worth watching for fans of the classic series. The episode was subsequently nominated for the 2007 Hugo Award for Best Dramatic Presentation, Short Form, an award that was won by the following episode "The Girl in the Fireplace".