I worked in a rural school. I talked to teachers during a school shooter drill, and more than half admitted to having a weapon in their car in the parking lot. It would take the nearest sheriff's deputy around half an hour to get there in an actual shooting. By that time it would be resolved for about 25 minutes.
Are they going to leave, get their gun, and come back? That is almost certainly illegal, and stands a very good chance of shooting the wrong person. That's why the gun is in the car -- it's not allowed into the school.
Their top priority is the children in the room, which usually means sheltering in place. If it were easy to get the children out we wouldn't have mass shootings in the first place. Are they going to leave the kids alone while they fetch their gun?
They may like the idea of playing the hero, but there is basically zero chance of that. There is no opportunity to get their gun and a billion problems with using it if they had it. It's a comfort blanket, not a tool of protection.
