As I said in the other thread, I think that would be a great idea until the first time a teacher snaps and guns down a student, which I can pretty much guarantee you would happen within a year (and that's being optimistic).
And look, I'm not saying I think that all (or even more than a few) teachers would snap like that, or that the same teachers would actively bring a gun to school with the intent of killing a student or students. What I am saying is that giving them a loaded gun in the classroom is a recipe for them to one day get pissed, snap, and use it on the kids they are supposed to possess the gun to protect.
There's a big difference between actively bringing a gun to a school with the intent to kill and having a loaded gun in your desk and using it in a moment of anger. One takes a certain kind of psychopath, the other simply requires seeing red for a few seconds and doing something incredibly stupid.