There should never be a reason for a teacher to carry a gun. They are there to teach not to shoot. If there is a reason to have firearms then that should be a security guard's option.
It is not always about mental health sometimes its just the culture or what goes on home. Like you said in another thread its human to be bullied and it turns some people into monsters. Teachers do need to protect themselves but having a gun is not the way to go. Someone suggested security personnel for that and I agree.