I don't get it

Armed guards in schools is almost certainly overkill. I could see specific districts with concerns doing it (they probably already have them, though), but generally it's not my cup of tea.

Nevertheless I'm astonished so many people think it's ridiculous as an idea.

Between putting armed guards in schools and tightening gun control, it seems obvious that the armed guards in schools would make these shootings less likely. Even if you like the idea of more gun control, I don't see how you argue with this.

Is it good policy?

Eh, probably not.

But these reactions seem to be more about peoples' emotional response to the NRA than common sense.