Some of y'all don't get how layers of security work and that's why this conversation is going in circles. Newtown shooter shot through the glass to get in the building. It changes that situation dramatically when the shooter can't get in the building, that's the goal here.
Would the glass be enough by itself, that's not the goal or the point, that was first thing he tried so if it was bulletproof he would've had to find another way in the building that he probably didn't plan for with school now aware that he's trying from hearing gunfire at the front door.
Door locks and bullet proof glass are two layers that really are worth the investment of not having more kids killed. Keep them out the building or at from going from room to room. When we are talking cost benefit, we have to keep in mind that one side of the chart is people dying, not servers going down.
Pretty much every mass shooting in this thread happened at a non-private residency, so I don't get why you even brought this up. This ain't mad max, but this mass shooting thing is the new norm, costing lives us trying to grapple with that, we're here now.