A lone policeman, armed with his service revolver is probably not going to engage a shooter. He is outgunned at that point. Normally they would call for backup
A single teacher, armed with a handgun, with little training is ill-equipped to do battle with an active shooter
If the opportunity arises, where he could get off a shot undetected, he may kill the shooter. But going head to head against an AR-15 is not adviseable
Mass shooters have been stopped by lone cops with their carry guns and/or single armed teachers.
There are people walking around today, lots of them, because mass shooters were stopped by such people way before "back up" would have arrived.
I didn't say it can't happen
But the expectation of untrained hero teachers is overrated
In the movies, the good guy always wins. In real life, mistakes are made....even by highly trained police officers
It sounded like you were saying that it never happened.
Maybe your belief that carry supporters are overrating the odds of a civilian stopping a shooter is a miscommunication on your part.