The hell he doesn't. The simple fact that he's there in a school full of kids MAKES him a target.
I've got nothing against allowing teachers and staff to carry, but that is not the only solution, nor should it be.
You are dreaming stupid dreams if you think an armed guard is totally aware 100 per cent and 360 degrees. You would need several. Why iot rurn the teachers into armed guards? allow them CC permits and advertise that fact. The bad guy then doesn’t know who he has to shoot first.