I'm not fundamentally opposed to allowing teachers who already completed the required testing/training for CCL to be allowed to keep their weapon in a locked drawer or something similar in their class.
I don't really think we should necessarily encourage these people to actively pursue the threat, but having a trained shooter in the classroom can help save lives. Again, I don't mean as part of an active engagement, but if I were locked in a classroom during this situation, it would be nice if someone on my side of the door was armed too. That could help protect the children who are locked into the room, helplessly waiting for someone to save them.
Do I necessarily think that solves all problems? No.
Realistically, if every teacher was armed and trained and each school had SOF operators as security, shooters would just pick another soft target (school bus, church, etc.). Making a soft target into a hard target only serves to divert the aggression towards a different soft target.