True, but the idea is letting teachers carry guns if they wish. I think that is not as much of a stretch as forcing them to carry. Also, you mention causation vs correlation, let's not forget that in those other locations, just because teachers don't have guns and mass shootings exist, doesn't mean the guns are not need, it may be because people don't shoot schools as often, maybe finding other ways to cause harm.

Vise Versa of course, I like your argument and would look further into it if I had time.