The same thing happend to my ipod.. except someone broke the lock off my locker and took my backpack and wallet and everything i cared about. that was about 4 years ago.
its either stolen or you dropped it in your locker, or you dropped it in the hallway, so you gotta check your locker, if not found then check your school's lost and found box in case anyone was nice enough to return it there and if not found you gotta go report it to your principal that an AHAsshole stole your Dsi from your backpack