It's no good trying any kind of AU if your story is literally 'wouldn't it be weird if [they were at high school/Boromir was an elf/Kirk had wings]?!'. You need a story.

The High School AU has the effect of placing all the characters on the same level. In Les Miserables, most of the characters never meet each other; several of them are dead while the others are still children. Many of the others connect in ways defined rigorously by their social status. So if you wanted to explore how Enjolras and Javert would get on if there wasn't a revolution on... well, you can't. Javert's a policeman, he doesn't talk to radical students.

But if you transpose it to a high school? Maybe Javert is some kind of prefect or something, and Enjolras is head of the Young Communists Society - but now they have to share classes, and have lunch in the same place, and get the bus in together. You can force two people who would never associate in canon into close proximity, simply by rewriting the entire background. ^^

(They would get on like a house on fire, of course - in the Pratchett sense. Flames, people screaming... ^~)

hS