This is almost exactly what I've been advocating for.
It says hardening doesn't work in cases when the shooter is a student at that school (which is most school shootings during school hours), though I think it's a good idea anyway, at least to the degree that our local schools have done so. Using a single door to enter and exit, locked to entry during school hours, ensure proper climate control is available.
It really seems to help teachers keep a better handle on things.
While more rare, the most devastating school shootings have been those in which an older kid who is not supposed to be there attacks younger kids. These are the cases in which hardening would be most helpful, and I think it's worth doing.