The High School AU is an Alternate Universe Fic that may or may not resemble the original universe its a Fan Fiction of, but the main premise is that most or all of the characters from whatever universe the story draws from are in an academic setting. Usually, this happens to be high school. It is (in theory) the opposite of the Self-Insert Fic- instead of putting themselves in a fictional universe, the writer brings fictional characters into a setting which they can write about knowledgeably, though some writers may insert original characters or even Author Avatars into this kind of reprised universe.