Poserworld has a lot of interior sets that could be turned into a school setting.

Poserworld also has a locker room = so does Trekkiegrrl here
I think Trekkiegrrl has a freebie hallway prop on her site as well.

Stringy sells an awesome school gymnasium set with interior/exterior. If not here, I believe it's at 3dcommune.

3dtoLife is JesterCGI - excellent model creator