Pretty sure that a major volcanic eruption is a really, really different scenario than an active school shooter. What is a normal response in once scenario doesn’t mean it would be a normal response in all other ones.
The people of Pompeii were suffocated in hot ash and would have been low to the ground looking for breathable air and possibly huddled together for comfort and prayer in the face of inescapable doom at the hands of the gods. No one to fight, no where to flee, so lay down and die together.
Soldiers under mortar fire often want to ...
a) stop moving
b) gather together.
Especially when they see someone get killed, and/or a mortar falls really close.
These are exactly the opposite of what people should do when under mortar fire. They should maintain distance and keep moving - hopefully to flank the bastards firing mortars at them.