Has a security guard ever put down a gunman in a school before he could kill anyone? From what I've heard they fail each time usually because they're taken by surprise or their gun is no match for an assault rifle.
If teachers were allowed to carry then they would need to be put through some kind of competency arrangement (Training) in place so there is no carelessness or should a shooter show up they don't start firing wildly and end up injuring (Killing) students rather than protecting them. Their competency should be inspected periodically.
Comments: If you want the teachers to carry guns then you will also want to do psychological testing on the teachers because some of those students can be so delinquent that a teacher might end up doing the shooting.
Exactly Kingowlbear, in the wild wild west I doubt the school teachers back then were packing why should they now when there is so many restrictions on how you can handle guns.
Robert you're making a point for consideration. This is not a solution for shootings. For me the main thing is to act on reports of psychos that make claims about killing.