I'm not being sarcastic. I'm just applying simple common sense logic to the problem at hand.
After all, this very incident shows that metal detectors and armed guards can't stop someone from smuggling a gun into a school. But when was the last time you heard of someone smuggling a student into a school?
Get rid of the students, and be default you get rid of the students with guns. You also get rid of any potential for students as victims. It's win-win.
As for the teachers, I'm quite certain they will happily sit in empty classrooms doing nothing. It's not that different from what they are doing now.