The arming of teachers isn't a good thing. As said above in today's society the consequences for a teacher ending a life in self defense would be horrendous.
What if a teacher is punched or assaulted, would the individual have enough restraint to not use a weapon in retaliation?
What if the student they killed is of a certain background?
Would a teacher be able to kill? It's one thing to point a gun at another person but to take a life?
What happens then? Is the teacher a hero or do they have to go through the court system and prove that it was necessary?
Again it's a system that in my mind just wouldn't work.