Bad idea, for the same reasons paramedics don't pack heat in the ambulance, and cops disarm before going to the holding cells.

I realize that the police are so bad that they actually help the school shooters now, but if we need an out of the box security solution, I'd suggest we consider arming the fire department before we talk about teachers. Fire department will get there first, have the tools to enter secured areas, and most importantly, are willing to risk their lives to save innocent people. And since they don't have 30 kids crawling all over them all day, there's less chance for an accident.