I think Jim Wright put it best: You'd be asking teachers to be trained and prepared to gun down not just kids, but kids that they know, potentially in cold blood. And that's some darker than black operations shit that we don't even train soldiers for, much less underpaid educators.
You can't "just shoot someone", even someone dangerous, on the drop of a hat. Modern military training (and unfortunately, police training) focuses on turning the action into a reflex, and the reason they do that is because people often will consciously or unconsciously "delope" and miss by a mile due to their ingrained inhibition against killing. People do this even in the heat of combat, in a war, fighting an enemy army that is trying to kill them. To suggest that this wouldn't be an overriding factor in getting teachers to gun down their own students is positively...Trumpian, I guess.
Personally, I think the pound for pound best thing we could do is improve conditions in schools to be more humane. That I think using the word "humane" in particular for school conditions is right says it all. The banality and daily frustration of the environment is more a contributing factor to shootings, and all other forms of school violence, than most people are talking about.