 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.