Overseas military schools had armed guards during the terrorist threats in the 80's (Libya bombing, for example). Schools were a prime terror target.
Gym classes going outside were accompanied, for example.
Kids never had a problem with the guards, and actually were nice to them, say, if guards had duty on an outside door and one of them stepped inside for a little bit to warm up.
I think armed guards would take more shit from asshole teachers then they would from kids. There are teachers who can't make the switch from being the 'boss' in their classroom to being just a regular person outside their classroom walls, in my experience.