So, someone armed with a gun or guns enters school and threatens to shoot students. Until that person actually shoots and kills someone, they should not be harmed? Perhaps if someone tries to physically restrain the person and is killed in the process then shoot? Any volunteers? But then just because the gun an killed one is no proof he will do it again, and shouldn't shoot in revenge, so, even then. Maybe hope gets tired or funs out of ammo?
At what point can you be reasonably certain that a person with a gun intends to kill and injure people? What is deadly force required to stop him?
With some schools using armed security or teachers; just seeing a person with a gun in a school is not cause to kill them.
You've put your finger on the hard question that needs to be determined at the scene in a very short time. But before you decide that CCW holders (an armed teacher or parent?) are incapable of making the correct decision, consider a few more things.
When a CCW holder has to make the shoot/don't shoot decision, he is often in a familiar location surrounded by familiar faces. He is often already in the location. A police officer is more likely to be entering the location in response to a call and may not know anyone there. These facts give the CCW holder an advantage in making the right choice.
The average CCW holder shoots better than police. For police, shooting is a small part of the job and they practice much less than the average CCW holder. Shooting is such a small part of police training that the FBI has found that even criminals shoot better than police!
I'm satisfied that those gun owners willing to take on the responsibility are able to make the decision correctly and are competent to carry out their decision.
https://www.forcescience.org/2006/12/new-findings-from-fbi-about-cop-attackers-their-weapons/
https://havokjournal.com/nation/the-average-civilian-pistol-permit-holder-is-a-better-shot-than-most-cops/
https://www.americas1stfreedom.org/articles/2016/2/22/on-the-front-lines/
In talking with the former police chief, and his wife, who worked for the sheriff's department, those who carried guns were tested every quarter. Because of budget cuts, the departments didn't provide ammunition for shooting practice; but they still practiced - buying their own ammunition to make sure they qualified each quarter to carry their firearm.
An officer at another police department was going through practice drills at the shooting range in the basement of the police station when he accidentally shot himself.
From these reports, it seems to me that law enforcement folks are continually in training and being tested on the accuracy of their shooting.
These were at larger police forces. In another town, the police department was two people. The chief had been a rancher in the area (and was a member of my congregation). There it might be true that someone who wanted to kill the police might be better trained and had more practice than the local police.
Another police officer in another town, was part of the police shooting team that went to competitions on shooting accuracy. He admitted that not all officers were as proficient as he was; but I doubt that there are many criminals who could shoot was well as he did. (He also admitted that the first time he shot at the target, he missed the whole target. He had to practice a lot to become as proficient as he became.)