The risk that a psychopath would purchase a gun and take it next door to shoot up a school, just because it happened to be in close proximity, is insignificant. In fact, I can't find a news article on such an event happening, ever. Schools do sometimes become targets for criminals, but not so much random targets. I understand fear of guns, but since they're nearly omnipresent in American society, I think avoiding a school on the basis of a nearby legit gun store would be irrational.
What you should be concerned with, if you're worried about guns, is whether a school has a good emergency plan in place, whether your child knows what to do in an emergency, and whether he knows what to do if he's around a gun.
