DaasYochid I am not sure how a rav can possibly determine, without asking them directly, if the people from the school (principal, teachers, rebbeim etc) saw the videos or did not, and either way, what was their reasoning. There may also be other info we are not aware of (e.g. there exist fake cameras, with a LED, which are sold as deterrents, sometimes people buy a few real cameras along with a bunch of identical dummies). If these points are irrelevant to the shaila, or if there is a way to know them without asking, can you please explain.
One might think that, regardless of whose money it was at the time it was stolen, the school is responsible, because the OP used the lockers as they’re supposed to be used, the school does not want them locked and being hiding place of unwanted material, so they request lockers are kept open and provide security cameras, they are in charge of security, and 50$ is a reasonable amount one can expect to be brought to school in cash, occasionally. If so, one may think, the school should shell out the 50$ and after that, it’s between them and someone else, if they should look at the video or not, and what to do. Instead they are claiming the OP is responsible for the loss, and so it seems to me it’s necessary to listen to what they have to say.