Been there... A few thoughts from an 'older' guy:
-Lock it up. Use a good key lock, not the cheap dial-type combo locks...those can be popped by putting a belt thru 'em & yanking. It might seem like a pain to take a key with you but your stuff will be safe.
-Don't leave things where they can be stolen...even in a locker...
We had to share lockers; the guy I was to share with was a real 'burnout'...as we called 'em back in the 80's. I never used that locker...I carried a LOT of crap with me or stashed it elsewhere.
I had my camera/lenses/etc stolen from the school darkroom; the door had one of those cheap dial combo locks on it. Fortunately it was on my dad's insurance so I wasn't out much. Even so, I NEVER left it there ever again.
-Don't talk about what's in your locker/car/etc.
My Stereo and amp were stolen from my car in the HS parking lot. I had been bragging about how great of a deal it was...
-"Friends"...
I had a 'friend' steal an $8 library book from me in Jr High. $8 is a lot of $$$ when you are 13. I found out several years later when I was visiting his house and saw the book. He thought nothing of it.
and FWIW, this was in a middle-class suburban area...