Seriously, though, I see no need for teachers to carry a loaded firearm. Their absolute FIRST priority is the safety of their students. And that means getting them into a classroom and locking the door, not drawing fire towards themselves or their students as a gunman sees them reaching for a weapon. AFTER that, they could go to a locked drawer and get out their gun to use if/when an intruder breaks into the room.
But what usually happens in a school shooting? Someone walks on campus and starts targeting random students and school personnel - i.e. whoever is unlucky enough to be in sight at the time. The chance of this teacher being close by, then ducking for cover, unstrapping her gun and coming up firing with a "kill shot" to stop the shooter is very, very remote. This happens in a Hollywood movie, not in real life. There's a FAR greater chance of teachers like these, who are licensed, but more likely to not have advanced training, killing students with "friendly fire" than stopping a gunman.