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The pilot was filmed in an actual school, and after the show was picked up, school sets were built that matched the original school almost exactly, and the rest of the series was filmed on these sets. The hallway set was actually just one "T" section of hallway, and it was constantly redressed to make it appear as if more of the school was being shown. The extras walking down the hall would pile up at the end of the hallway after every shot, as there was literally nowhere for them to go.
Series producer Judd Apatow told Vanity Fair in 2012 that whenever he sees an opportunity to use anyone from the show, he does. It is his way of refusing to accept that the show was cancelled, and that all of his subsequent movies are the continuous adventures of those characters.
The creators were determined not to end each show with a typical "happy ending". One notable exception is the pilot episode, which the creators purposely wrote as a self-contained story, in case the show was never picked up for production.
The actors improvised lines in many of the scenes to achieve a sense of reality.