Still have a problem with armed teachers. They have enough duties without the continuous training required to be effectively prepared for combat.
That's why you don't require all of them to do it. You only allow those who are already CC permit holders and who are willing to put in some extra training in how to effectively guard the one door into the classroom. IOW, I'm talking about the teacher who gets an alert that an active shooter is in the school, puts the kids behind whatever barrier(s) he can find, pulls his weapon and watches the door. You're talking about urban combat training wherein Dirty Harry stalks the hallways, snapping off hip shots and snarling, "Do you feel lucky, PUNK?".
This isn't rocket science. There's usually ONE fairly narrow door through which a shooter has to enter a classroom. An alert, armed teacher has a pretty good chance of stopping him at that point. An unarmed one has none.
And, how many armed teachers does it take for a sign to go up outside to the effect that there are armed teachers on premise? Also, which school is more likely to be attacked in the first place, one with such a sign or the ones with signs that proudly proclaim there are no oppositional weapons on campus?
I can tell you which, because the anti-gunners even understand the simple truth. Not a single one of them would post a sign outside their house proclaiming that their house is weapon free.