I understand your rationale here, but for insurance companies the term "not liable" is a very gray area. An insurance company would be obligated to pay for the legal defense of a municipality, a school board, or a school employee in such a case, so there would be enormous costs -- and maybe even a large out-of-court settlement -- even if the insurance company's client is technically "not liable."
I know this is a different state but,in Ohio at least, you have to have security at any event that serves liquor, usually an off duty cop, however lefties balk at armed security for a school? Typical liberal thinking!