The teachers would not have to be fully-trained LEOs — no where near it. They would mainly be a deterrance — or, failing that, they would be reacting defensively to a situation in familiar surroundings. They would likely train for different scenarios.
Reservists are routinely sent to foreign wars; where they might face a far worse enemy. Enough teachers should be able to qualify to bear arms.
Psychopaths respond to fear.
If they are afraid, they stay away.