Besides many of the issues raised, there's a pretty horrible one I can think about. While they may not use it, so many teacher in really bad public schools get fed up when they are not respected by errant students. They might assert themselves with fear by the fact they have a gun.
Even if it was for protection, as a student, (especially when I was in high/middle school) I would not be able to learn comfortable or forge a working relationship with my teacher when they are armed and in a authoritative position over me all the time. It's frightening.