I'm going to have to disagree with Magyk on two points.
1. We aren't getting invaded because we have guns. The reason no one wants to invade us is because we have nukes. We could strip down our military just to nuclear subs, missile silos, and some bomber/fighter squadrons and we still wouldn't be invaded because we'd nuke the stupid bastards who'd think about trying it. Warfare has changed big time with the invention of nuclear weapons but the way we think about it still needs to evolve.
2. More on topic, I have to disagree with Magyk because, even though I consider myself more of a moderate overall and even liberal on a few social issues, I don't agree with banning any weapons, even snowtrooper rifles. What I do believe in is a little common sense. There should be no gun show loophole. Everyone should be checked out when buying a weapon. Those who want higher powered weapons should be checked out even more. (Whose idea was it that you get the same treatment for a little pistol as an AK-47 anyway?). With mental health checks and updates and people coming to your home to see that you have things properly secured if necessary, for the most destructive weapons. I have no problem with people owning snowtrooper rifles but they damn well better not let their mentally ill relatives have access to them.
Moving on, I'm not particularly keen on arming our teachers with hand guns. The logistical problems of identifying who the real shooter is is one reason I've already mentioned. Another is because I know if we allowed every single teacher to carry then eventually we'd have one that went postal. Also, some kid would eventually get hold of the teacher's gun that the teacher forgot to secure. My recommendations for school security are as follows:
1. Equip teachers not with guns but with tasers. These aren't as likely to cause fatalities if abused but would provide a reasonable amount of defense against these lone shooters. They are excellent disabling weapons and can be used effectively to ambush shooters.
2. There should be at least one police officer stationed at each school building for security who is allowed to carry guns. (Preferrably two, in my opinion, so that there would be backup in the event the first officer is targeted initially and neutralized, but I know certain localities would be too cheap to pay for that).
3. Entrances to school buildings should have bullet proof glass and drop cages for security(Entrances only. Not the whole building as, again, that would be too expensive). The goal in doing this is because it is imperative to slow down and/or contain any shooter long enough for assistance to arrive.
These are some reasonable security measures that have limited risk of being abused and would provide ways of delaying shooters and minimizing casualties.