Striking defenseless kids is all about striking at control mechanisms. Teachers, administrators and teh nooz are the intended audiences.
Massacring kids in a classroom fits an objective in one of these people's minds... massacring kids actively fleeing and shooting them in the back transcends the mental boundaries these people have constructed. They have power in a classroom, they have leverage, they have an audience. When the kids are fleeing, they aren't paying any heed to the shooter other than being immediately revolted by his presence and trying to get away. The shooter loses his enforced "charisma" as a hostage keeper, and has no means to maintain control over his audience.
The kids are FAR safer running through the hallways, running away from an active shooter situation, IMO, than cooped up in classrooms.
The huddle-in-a-corner-with-the-lights-out response sickens and revolts me. It's herd mentality practiced by prey societies. And it's a sick accounting of all the bodies in our litigious world, even if the bodies are room temperature... rather than possibly having kids escape campus and be unaccounted for, for a brief period for the rest of the day as they seek safety elsewhere than on campus.
Gotta have CONTROL above defense and safety.