DaasYochid I am not sure how a rav can possibly determine,…
If the rav thinks there are relevant questions to ask, he will. Most rabbonim are not, in fact, experts in Choshen Mishpat, which is why these hotlines, under the auspices of well known and respected poskim, are so valuable. Obviously, these poskim (Rabbis Kohn and Forst) feel that there are situations in which one may ask a shailah anonymously, otherwise they wouldn’t allow it.
the OP used the lockers as they’re supposed to be used
To keep cash? Doubtful. At the most, it would make the school (if such an entity can be held responsible) a gorem (indirect causer) and not chayav in beis din. And I don’t think it would even be a grama.
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.
Indeed, ask a shailah and don’t rely on anonymous posters.