A really bad idea.

1. In case of a truly violent situation in which a firearm is not involved this could simply lead to the person who is perpertrating the violence becoming armed.
2. Accidental discharge of weapon becomes very dangerous. Even police officers have accidental discharges. There's a groove in the floor not 5 feet from where I am typing from an accidental discharge made by a police chief (my father).
3.Not all teachers are stable. Teachers do get out of control and have been known to perpetrate violence on occasion.
4. Re-enforces a might makes right mentality which could lead to aggressive acts in and out of the school.