I agree that one person with a pistol is going to be at a deadly disadvantage trying to close with and shoot a person with an assault rifle. He'll probably lose something like 9 out of 10 times, or worse. What you want is SWAT guys, with smoke/gas grenades, gas masks, and assault rifles of their own. And you want at least 3 or 4 of them. A team like that would have decent odds of getting the guy without one of them getting shot.
So, we don't want armed teachers. We want a SWAT team in every school. So let's say we do that. Surprise! Suddenly the trend is crazy killers shooting up churches.
If the solution is SWAT teams, we're going to need them in schools, in churches, at theaters, concert halls, stadiums, in all our workplaces, restaurants, nightclubs, malls, parks, and in the residential areas.
Doesn't sound workable. Trying sounds like a way to turn society into a prison, all gates and checkpoints and guards.
I favor
1) making it harder for crazy killers to get guns, and the more deadly the gun the harder.
2) working harder to find crazy killers before they go off, and intervene.
3) Working tremendously hard to build a decent society characterized first and foremost by loving kindness, where the situations that twist people into crazy killers are abated, not fueled, by the prevalent memes and norms.