Japanese films set in high schools are now about as common as cherry trees in Tokyo. This makes box-office sense: Japanese teens read a lot of manga about kids their own age and the more popular become fodder for films.
In manga anything goes — from time travel to alternative universes — and that has carried over to the movies based on them.
This trend has not been all bad: Freed from the need to come up with something fresh to say about actual high school life — which is like coming up with something fresh to say about cherry blossoms — filmmakers have used fantasy academies to comment on everything from national politics ("Teiichi: Battle of Supreme High") to city-versus-suburb status anxiety ("Fly Me to the Saitama").