Question: What happens when the armed teacher goes on a rampage? Do we arm the students? The custodians?
Further question: What happens if the teacher decides to go on a rampage even though he/she isn't allowed to carry in school?
Shooting up an entire classroom isn't exactly something done on a whim, where you get angry and then cool off after going home and thinking about it. None of the school shootings in history have been "I'm angry at this classroom that's right in front of me and I have a gun, so TASTE MY WRATH."
If someone decides to go on a rampage, they're going to do it even if they're banned from carrying in that area. If they don't have guns, they'll go like the Akibahara killer and just ram a truck into a crowd and start stabbing people. I'm willing to bet that far more people (yes, even in America) have vehicles and a large collection of melee weapons than ones that have guns. Stopping gun violence has never been achieved or even SLOWED in America by preventing the non-crazies from carrying weapons in public. The problems aren't related to the guns, but to the society that allows these people to get to the point where they feel that murdering their colleagues or random people is the only solution or something that they otherwise desire, then trying to blame everything on guns instead.
