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.
Because they're teachers, not armed guards. And if you don't think the students know who among them would and would not shoot, you're the one with stupid dreams.