Armed teachers?

There are a lot of arguments against President Donald Trump’s recommendation that schools arm teachers to ward off a potential mass shooter.

A couple of the better ones are:

In the chaos of such an incident, anyone with a gun inside a school building when lawmen arrive could be mistaken for the perpetrator and shot, including an innocent but armed teacher.

Also, would a teacher really shoot to kill a student whom the teacher knows? Teachers love their students, including the troubled ones. In the crunch, could they really turn that emotion off?