I'll boil down the argument for you, so you can really understand what I'm saying. I'm NOT saying that a teacher CAN'T stop an armed shooter, not at all. But here is the argument:
YOU: an armed teacher can help stop a school shooter.
ME: that may be, but an armed shooter in schools is extremely unlikely, and an armed teacher would be MORE LIKELY to harm himself or others, than stop an armed shooter. Therefore, arming teachers is a disproportionate response to a minuscule risk.
The fact that I've already shown you people who are licensed and yet were still irresponsible and harming others in a fucking university, but no armed teachers have stopped a school shooter, suggests that my scenario (someone armed in a school hurting by-standers) is more likely to happen than your scenario (someone armed in a school preventing another armed shooter).