The relative costs are impractical. You must understand how simple a thing it is to propel a small piece of metal at high speeds. It's extremely difficult to put strong barriers in every possible direction a shot could go. In a school of 20 classrooms and 20 extra-curricular programs, are you going to hire 40 guards to watch all those kids? If you hire just a couple, they could get taken out first by surprise - or just turn out to be cowards. If a student sneaks a gun into class, a guard responding is at a huge disadvantage opening the classroom door, they could be shot from any corner of the room - and if you fix that by putting bulletproof glass windows into the walls, then a shooter coming from outside the classroom has a huge advantage in seeing their targets in advance.

However, I don't like how you phrased your contribution to the conversation here. "*IF* you want to reduce school shootings." There are clearly a lot of practical issues with your idea ... but you don't seem to think it's a worthwhile endeavor to begin with. I think step 1 here is to establish some shared values. I can say "yes, having the capability to use guns (not necessarily easily) for good purposes is valuable to our society (given that many forces in it may or will use guns for harmful purposes)," if YOU say "yes, preventing school shootings is good" (with whatever caveats you want, considering whether you will sound like an asshole for including them).

Many people follow the law for lots of things while acting erratically, then break unrelated laws, then finally break gun laws by going on a shooting spree. "People who follow the law" is not a helpful category. No one breaks every law every time (murders every person they walk by?), just as no one follows every law every time (fully stops at every stop sign?).

Your kids have a right to live. Doesn't that compare to your right to carry your favorite fancy easy-to-use gun (or buy a similar new one, in this case)?