I've seen and recently heard of accidental shootings and realize that they wouldn't kill masses of people, but still have seen in the news and heard about other instances where people were injured or killed by gun accidents.
In the past week, a guy bought a handgun and "unloaded" it in his car by removing the magazine, then apparently dropped the hammer on a loaded chamber, shooting his son in the car, minutes after buying the gun.
In another incident in Maine, there were very "proficient" police firearms instructors examining a handgun behind the shooting line when it went off and shot the Sheriff.
My B-I-L, a very competent match handgun shooter and firearms instructor, in separate incidents, accidentally shot out his TV, a sliding glass door, a picture of Abe Lincoln, and a neighbor's dog. Before the dog incident, he had a concealed weapons permit.
I say we need to proceed cautiously when considering arming teachers. There are too many young kids around and if a teacher were to accidentally harm just one, it would be as devastating to that teacher and almost as devastating to the community as if there were a mass shooting.