Reality is that unless you're a vet and have actual experience in combat or had experience as a cop in firefights, most people would shyt their pants and not be John wick in even less extreme cases such as taking on an armed home intruder who himself does not want to die and may just be armed with a pistol .
In cases like this is a different ball game when you have a dude who doesn't care if he dies and is on a suicide mission to kill everyone on sight and is armed with an AR 15 . I remember reading that many police officers are hesistant to engage situations where the person is armed with an AR or an AK and instead wait on backup because they are overpowered.
So more than likely if you armed teachers, you may have some success stories where the teacher won out but I think it will be rare and more commonly you'll find these people will just focus on running for their life instead of going toe to toe with a psycho .most people aren't built for that and it takes a rare person to stare death in the face and not just immediately start running .