That's so stupid! Every aggressor would shoot the teacher first.
Does violence in the American society really increase? I cannot know, I'm in Europe. But if it does, arming more people is counterproductive. Guns may help to defend for a while, until everybody is armed. Then everybody will need bigger, faster, more-deadly guns to defend against the people with normal guns. But soon everybody will have those bigger, faster, more-deadly guns... and so on... and so on... but they'll never find out:
What complex of social changes causes the increase of violence in the USA?
As long as nobody seriously tries to find out what is happening in your culture, you cannot do anything. Do you want a whole war equipment around every classroom? Fine, that will teach your children that violence is the solution to everything. Surveillance, control, counter-aggression, surveillance, control, counter-aggression ... the solution to every social/cultural problem, great, that's your future. Have a lot of fun!
Sadly, if you go on like that, one day you might need foreign historians to find out what the h*ll is happening in the American culture right now that makes young people more and more aggressive?
You don't have a problem of more/less guns. You have a complex set of social and cultural problems that nobody wants to see.