With all due respect, this is a silly idea.
I spent 20+ years of my life in and around education. In all that time, I might have met 10 that I would feel safe knowing they had a gun on them. I have had teachers that I didn't feel safe around UNARMED.
In addition, this would simply get guns into school FOR kids to use. Teachers are not going to tolerate an ankle-holster, or anything else, no matter how out-of-the-way. The teachers I had would simply throw it in a desk drawer for the deranged in their class to seize and use at will.
Then there's the sticky business of "suicide by teacher", or teachers abusing their right to carry the guns themselves. We can't keep teachers from screwing 14 year olds, why should they be any more responsible with guns?
Pilots, sure. There's no reason for them to be interacting with the passengers, they are locked in the cockpit, and they wear a uniform that could accomodate a firearm. Teachers? Pfft.