It has happened. It's also a possibility a person has to take into consideration. But do you think this negates the times when a CCW holder has been able to stop a shooter? Not everyone is geared to do what needs to be done in these situations, the guard in the Parkland shooting is proof of that.
Interesting that you feel teachers should not be armed, if they so choose. Those that would choose to do so, already have their CCW in life outside of school, it just expands it, if they so choose. No one needs to know who is or isn't carrying, just like a day on the sidewalk, you have (or should not have) any idea who is carrying or not. And that helps, because, does the bad guy really want to take that chance?
"gun free zones' are like shooting ducks.... you know that no one is going to shoot back, so easy targets.