Our school (who wasn't actually considering it) said the insurance company would drop our coverage if we allowed armed teachers on pemesis. Honestly, I think I'd quit if I knew someone on staff was concealed carrying. We deal with a lot of shot, but the last thing I want between me and a troubled kid is a gun.
Teacher here. I consider all of the teachers I've worked with, and I consider how few of them I would trust with a loaded firearm under the best possible circumstances (maybe 10 percent).
Then I consider the number of those who would actually consider using a firearm (down to about 3 percent).
And then I consider how many of those I would trust with a firearm in a room full of students... and it's zero.
I finally just avoided the problem by moving to a country where civilians are not generally allowed to own guns and school shootings are not a problem.