No, it's not like teachers can't go crazy. I mean there are horrible people that molest kids out in the world, what's to say a teacher couldn't go crazy and harm students and other teachers with a gun? It's not just kids that go crazy. I don't believe in having any weapons in a school. I can't see a lot of parents liking this and the teachers themselves would most likely be extremely uncomfortable being trained and certified to user firearms and even having one in the classroom. Honestly it would end up doing way more harm then good. All schools just need more security measures in place to prevent shootings, metal detectors, cameras, security officers and people looking out for suspicious behavior from anyone.
If someone seems off or something like that, it should be reported and investigated. You just never know who could snap like that.