That isn't actually so, in general. In the US, for example, many places where guns are more readily available (small rural towns in wealthier areas) have fewer homicides - especially gun homicides - than places like inner city minority neighborhoods where proportionally many fewer people keep a gun handy.
In the US, gun ownership is positively correlated with income - guns are fairly expensive, and mostly recreational - while homicide (especially gun homicide) is negatively correlated with income.
Michael Moore's much-misunderstood movie "Bowling For Columbine" goes into that at some length, for example spending several minutes interviewing Canadians about their rate of gun ownership (high) and murder rate by gun (low), and the main issue that seems to come up in the interviews is that Canadians are not fearful of their neighbors, in general.
I would think guns in a school would be a hazard, if not from the well-known psychiatric oddities of the teaching profession than because schools are full of children and children do dumb stuff - perhaps some regulation, at least, would allow teachers to protect the precious children from the dangers that threaten on all sides without creating too much opportunity for childish mischief itself to become lethal: one might for example require that the weapon be a full size shotgun or similar and the shells kept on the teacher's person.
