RBBM. Many teachers have saved children by hiding them. One shot is no guarantee to take out the shooter. In a chaotic situation, you need to be able to get a clear sight of the shooter, who doesn't have a clear sight of you, hoping they don't have a bump stock, or a high capacity magazine, so they don't just spray the area and take out yourself, and anyone else around, and then be able to hit them. Meanwhile the teacher IS going to be worried about the children they have left alone. How does that teacher know they haven't left a classroom full of 7 year olds, for the shooter's accomplice to find and massacre?