The basic concept isn’t a bad idea, assuming it works and you already have the cameras. If it ran locally at a negligible cost, I’d say it would be a potentially useful tool. But even then, it wouldn’t solve the problems, just help identify them more quickly, especially in situations where a gun is in the school but isn’t being fired yet. Less useful for an active shooter, more useful for spotting someone flashing a gun, brandishing, showing off to their friends, or even selling.

It would be like a smoke detector, good to have but only as part of a larger plan, and also not something you should be dumping a ton of money into.