People need to train both hard skills as well as prepare themselves mentally for the horror they will encounter on scene. Imagine responding to a school where there are dead children who've been shot right in the face and others are laying around screaming and covered in blood. How do you predict how you will react? The average American has never seen a body that resulted from violent death and even law enforcement who have seen it often rarely see dead children.
People gotta get into the right headspace for dealing with that as well as dealing with the fact that the killer most likely will be another child that you will have to shoot.
Can you walk past a wounded child who will most likely bleed out or do you press forward toward the threat?
The idea of rushing in and taking out the shooter sounds simple but it really isn't unless you have prepared with alot of mindset and physical training. Law enforcement and armed citizens alike need to really step it up because luck has nothing to do with it.
Active shooter training, at least the exercises I participated in and conducted covered these these things. Officers responding had to pass role players who were portraying dead and wounded children and other officers. This kind of training became standard about 2000 at least in our area. We had multi agency exercises because any active shooter incident in our area would have been a multi-agency response. I don't know what happened in Texas, my thoughts on it would be off topic here.
These things are something everyone who thinks they might be in a position to deal with an active shooter needs to wrap their head around before the situation arises. It's not as simple as it looks on TV.