The claim I am making is not that locked doors will stop a shooter from shooting anyone. I am saying that locked doors will make him less effective at shooting

The claim I am making is not that locked doors will stop a shooter from shooting anyone. I am saying that locked doors will make him less effective at shooting many people. Once someone decides they're going to take a gun and start killing people, it's non-trivial to stop them. That doesn't mean that we can't reduce the number of people they can shoot at and kill. That's where closing off the avenues that a shooter can take is effective. And even if someone did wait for classes to let out, firing a gun and killing people at a distance is not easy. You start shooting at people, and presumably they'll scatter.