In a High School AU I think this is more like how Dr. Cid would be.
(SPOILER)

Anyway...my thoughts.

Cloud should be a teacher. He's whopping 21-year-old. He is already halfway to retirement by FF standards. He's the Gym Teacher I reckon but not a very good one. He means well but he just doesn't have that ALRIGHT MAGGOTS gym teacher voice. He is popular among the kids for his laid-back personality but he is affectionately mocked behind his back for saying "let's mosey" when calling people out of the locker room.

Tifa would be Gym Teacher for the girls. She's about the same as Cloud but instead of mocking her for her speech patterns all the girls are jealous of her....yeah.

Classroom rumors abound about what. Mr. Strife and Ms. Lockhart get up to.

Ynna will be an all A-student, 4.0 GPA and whatever else because of the pressure of her parents and big brother Wakk and big sister Lulu. (kindly pressure but pressure nonetheless) However when she's out of school and with her friends Rikku and Paine she gets up to all sorts of mischief.

Rinoa is the Prom Queen and everyone from Tidus to whoever else hits on her but she takes an interest in that sullen loner boy everyone just avoids People think he's in a gang or on drugs or whatever but he's really just not very sociable.

Ashe needs to be a teacher...of what I don't know. History or Politics maybe.