School district's plan to stop shooters: Arming students with a bucket of rocks

I think a shooter can just run behind a desk to block the rocks then poke the gun out to shoot. I agree doing something so that they are not passive targets. But I wonder if there is something better. I think high powered mace/pepper spray foams and bombs. Maybe the chemical agents with the rocks will do it.

If only someone could invent a device that throws small metal rocks at high speeds...

Is this a cheaper substitute for having armed guards and metal detectors or a preferred tactic instead?

At first glance, and as a last line of defense, that is a less bad idea than many others.