Question About High School AUs

It goes without saying that something can't be a High School AU if the original work is set in high school. But what about if the original work is partially set in high school, like if there were flashbacks to when the characters were in high school? If a fanfic or an adaptation decides to focus on the scenes set in high school, would that count as a High School AU?

Hmm... inclined to say that that would be a fic set in the past, and not a High School AU per se. That's more for works where 'the characters are in school together' is very far removed from the original premise.