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.
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.
About halfway through the season, it was evident that the show was in danger of being canceled. The rest of the episodes were put on hold, while the creators wrote and filmed the final episode (which was written in a way as to serve as both a series finale if the show was canceled, and a season finale if the show would've gotten picked up for a second season). Several episodes later, it was canceled.
While the show took place in Michigan, it was shot in California. Outside scenes proved to be difficult to shoot because no palm trees were allowed to be seen in the shot. If you look closely you may see at least one.
Nearly everything that happens to the characters happened to Paul Feig or other members of the crew - for example, the blue suit that Sam Weir wears in "Looks and Books", the showering in the locker room storyline in "I'm With the Band".
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 canceled, and that all of his subsequent movies are the continuous adventures of those characters.