The only way to stop a potential mass shooting from becoming a actual mass shooting is to shoot the attacker. That can’t happen if only the attacker is armed. Mass shooting will stop quickly if there is a string of them ended in the shooters taking a dirt nap.
Who gets called when these things happened? Armed cops. Since they’re usually not on scene it gives the killer plenty of time for whatever they want to do.
Having someone on site is the only solution. I’d compromise and say anyone at the K-12 school that wants one has to do even more training than a CCW in Texas. Even psychological exams and so on.. They should also get a yearly bonus for it as well.