Not appropriate for teachers.  If a school wishes to employ security guards, those would be the people to carry a weapon.  It just doesn't seem right for it to be the teachers.  Even if they have a permit and training and all, it's not the right role for a teacher to have in the school.  A security guard is understood to be a trained professional, so if they carry a gun, everyone understands that the reason they have it is to protect the students and staff.