@Lash,
Self defense comprises fewer risky situations than police work. Someone who is defending themselves doesn't go out to confront the bad guy. They only fire when the bad guy comes and attacks them.
But yes, a defender can make mistakes too. If a teacher has to fire on a school shooter it is already a bad day no matter what.
But probably not as bad as the day would have been without any ability to fire on the attacker.
I'd recommend one of those quick-action gun safes in the classroom. No hiding the gun. One of the students will inevitably notice, and thirty seconds later all the students in the school would know. So I'd suggest don't try to hide it and just rely on the gun safe to prevent access to the gun.
I also recommend a centerfire rifle. Portability isn't necessary. Accuracy and the ability to punch through Kevlar is necessary. Probably something appropriate for deer hunting. A lever action .30-30 would be a pretty good choice.
Keep in mind that while rifles tear through Kevlar, they don't punch through steel plates unless you have AP ammo. So if the shooter has a steel plate you'll have to aim somewhere else.
I also remember a proposal that someone made after one of the shootings some years ago, not to arm the teachers directly, but to have a rifle in a safe in the principle's office. So if directly arming teachers doesn't happen, then maybe that would be a good option.