It seems to me that in schools, and specifically in that school, it is customary to collect such amounts for gifts to teachers in a happy occasion, and for similar amounts. I do not think occasionally keeping 50$ in a locker goes beyond normal use. Also, it seems to me the responsibility of the school is not indirect, but there is more to it: the school is somehow aware of the possibility / likelyhood of thieves, that’s why they have cameras, yet they demand the lockers have no locks. This is obviously their choice and their convenience, as they might instead request all students who choose to put locks, to give a copy of the key and its locker number, which then is attached to a keychain along with all the other keys, and when not in use, are kept in a safe place. The lockers being open are not consequence of an intrinsic fact such as the school wanting to check the lockers which we may argue is a part of chinuch, but instead, it seems to me it derives from their convenience to have them left open, and not wanting to be bothered with keys.
