Way to go America.
Lets combat gun violence in schools by throwing more guns into the mix.
What happens if a teacher flips out and shoots a kid?
You could fly to America, buy a gun illegally and start shooting people tomorrow, what happens if you flip out and do that? What happens if someone in the UK Armed Police flips out and starts shooting members of the public? What happens if someone in charge of an AH-64 Apache Longbow flips out and starts firing Hellfire missiles into the center of London?
I don't think school shooters are put off by the idea of possibly getting shot. I think arming teachers will make them the first target and also create a holy grail opportunity for kids to steal something worth money, is dangerous and is cool.
They are put off because it makes it harder for them to achieve their goal which is to kill as many people as possible, that's the deterrent, being stopped, not being shot. They've probably accepted they're going to be shot by Police anyway.