You would need the teachers to pass police and psych tests before being issued with a gun,
“Didn’t the deputy pass police and psych tests?”
Yes, he did, and the tests were correct: he did not go on a killing spree like the shooter, but he was too sane and sensible to risk his life facing a young man armed with an Assault Rifle when he only had a police-issue pistol. Psych tests don’t test for bravery, only for sanity. There’s no evidence that Deputy Scot Peterson had any mental Health issues at all, just that he exercised excessive caution during the killings. It is possible that he is not a coward: some people freeze in their first combat situation; it is common in warfare for some people to be incapable of taking action against the enemy. The problem is that we expect armed police officers to confront and shoot armed offenders, and it is disappointing when they don’t.
The police and psych tests for the Teachers are the same: they will stop us giving guns to mentally unstable teachers, but we can’t predict how a given Teacher would react under fire. That’s why more than one Teacher at each school should be armed. If you had four armed Teachers, it is almost certain that one of them would do the right thing.
“The entire NRA-sponsored “good guy with a gun” meme suggests that if the whole country were armed, there would be far less violence.” I’m arguing from what I have reasoned for myself, not following an NRA script, and I’m specifically considering mass-shootings at Schools, not general homicide statistics. Arming the Teachers is something we can do now, and would reduce the risk to school children from serial killers.
You are probably right that less guns means less shootings, but the point of the Second Amendment is not safety from homicide, it is safety from Tyranny. The founding fathers believed strongly in limiting the power of Government, knowing that a Government strong enough to give you everything you want is also strong enough to take from you everything you have. The right to bear arms is specifically to allow Americans to make war on their own Government should it descend into Tyranny. It is designed to keep the Politicians focused on serving the Public, rather than ruling the Public.