C'mon, man. If there is one thing we all have in common, it's that none of us want children to be executed by bad guys. We can start there.
And top marks to the football coach, but that's one data point. He may have stopped and shot the bad guy, but put handguns under the windbreakers of a thousand football coaches across the country, and it's not hard to imagine a lot of unintended consequences. This coach cracks under pressure; that one has his go off by accident; others get theirs stolen by a student, or pull it for a minor altercation, or focus on being Dirty Harry when he should be guiding his kids out the back door.
Teachers, and in this case coaches, are famously overworked and stressed. Some of them may have been Marine snipers in a former life, sure, but taken collectively, I don't want them having to juggle firearm safety and counter-terrorism duties on top of the mountain of others they already have.