You have a set of characters. They work well as an ensemble; so well, in fact, that they can be slotted into just about any scenario you care to imagine, within the constraints of genre (fitting an ensemble into a different genre is a completely different trope). So you can see them, identical but for different trappings (he was wielding a sword, now it's a blaster pistol...), in places as diverse as Feudal Japan, the Modern Era, Space Opera, etc., etc., etc. This is one of the essential justifications for Transplanted Character Fic, including the High School AU.