I worked as a teacher's aide in an inner city school. It would be nearly impossible for a teacher to keep a gun scure on their person at all times. Eventually one of these guns would end up in the hands of a student and it would end badly.
Ask a police officer. Occasionally their guns are grabbed by bad people off their persons. They know the stories about what happens then. Now think of a school where a teacher might have 30 or more children in a classroom they need to keep track of.