When my son was in high school a couple of years ago, the students were instructed that all of them were to sit in a corner of the room if there was a shooter on the premises. All of them! I told him to screw that. That he and the rest of the boys should stand ready to throw whatever was handy at the shooter. Their backpacks were filled with at least 20 pounds of books. That would be a good start. Additionally they could throw the chairs and desks. That would slow down any shooter. But imagine if the shooter was a student who knew that everyone would be piled into a corner - fish in a barrel. How stupid are administrators in schools?
Certainly if a few charge and a few throw desks, the chances that the shooter could harm more than a few students are lessened considerably.
If a few students are throwing chairs, the shooter might duck one and get some shots off, but he probably won't be able dodge every one. Cowering in the corner will mean easy pickings for the shooter.