Teachers should NOT be allowed to carry in school. The very nature of a teacher is a detriment to the action which may be called for in an active shooter case.
There are plenty of qualified people such as retired police and veterans that would be glad to post up at a school, probably for free a few hours a day.
For those who are uncomfortable with guns there are other means of self-defense and strategic planning to prevent something like the recent shootings. My wife carries a Tazer C2 (shoots the darts 15') through the parking lot at the hospital she works. Pepper sprays inside the hospital are not welcome. The teacher could activate a flash-bag & then tear gas. All not lethal with little risk of permanent collateral damage. Have a panic button to turn on the sprinklers might even help to fog up the room. These are just brainstorm ideas that may or may not work but I would think there could be may others better than cowering in the corner waiting to be shot.
The threat may not be from gunfire. Firebombs, pipebombs, bladed weapons. The way the schools are now, the most dangerous implement for self-defense is a pencil or fire-extinguisher.
Posting qualified CCW permit holders at school who qualify on a range regularly but don't have the power of arrest. That way they don't get carried away and chase the perp down the block or trying to be LE. CCW is all about defense, not offense.