The idea NOT for the teachers to walk around looking for the hostile shooter. The goal would be to first secure the classroom, lock the door, take a defensive position with your students behind you, and guard it until police arrive. I can't think of why this isn't a better solution than waiting helplessly for the shooter to break in and spray bullets at people.
The thing is this subject will always be open to Monday morning quarterbacking. Suppose a teacher did end up killing an innocent while trying to defend his classroom. Are we going to scream about how arming teachers was a horrible mistake? How can you know if despite the 1 child, 17 others were saved?
Also not every teacher should be armed. Only those who feel confident enough to take up that role.