Can't arm teachers. They're "Union Thugs, hellbent on corrupting our kids and bankrupting our cities." remember?
If you want to do it right - multiple security officers, armed and armored. A building designed to the purpose. Monthly, or at least quarterly response drills. A well implemented consideration of what's an actual defense arm, what's a hunting arm, and what's only really useful for paranoid fantasias and that little kick of adrenaline you get from things that go boom might be useful not as prevention, but as mitigation of lethality. But that costs tax dollars, time and political capital. It costs having a sane discussion without preconcieved notions.
We'll have some theater, a little political masturbation, and a lot of defense of the fantasy of killing waves of governmental commie-nazi-fascist-hypno-druggie-gangers, some anecdotes about shooting bad guys, throw up our hands, and ignore the problem for about six months, at which point some teenager will come into school with daddy's P228, kill 6 people and wound three, and we'll do it all over again, because we can't spend money on schools, we can't spend money on teachers, and we certainly can't discuss what's appropriate armament for a civilian.