luke:
you have not "bought" the lock you have put down a security deposit on it. if the agreement (even verbal) is that you'll return it for cash back it is not yours.
the understanding there is that if the lock fails through normal use the school will replace it. if you're expected to return it at the end of your tenure there, then they still own it, and it's not yours to abuse.
if the retail price of the lock is comparable to the security deposit on it, then you could realistically purchase another one from the school w/o the intent to return it.
if, however, the purchase price is higher (probably) then you'd be intentionally exploiting their lock loan system, in that you're choosing to steal value from them.
personally, i'd go to the janitor, and tell them that you'd be interested in buying any locks they have to cut or that broke for 5$ or 10$