Simply having a weapon available is not enough. Whoever has a weapon has a responsibility to go to the shooting range regularly to develop and maintain their skills. Also, they must have a mindset that they are willing and able to take a human life.

Also, what if a student steals the gun? Students often steal from teachers. They are at a stage where they rebel against authority, and they see the teacher as an authority figure to be resisted. To their minds, stealing from a teacher is a statement of freedom. I know; I was a teacher. At various times, my belongings were stolen, my vehicle vandalized, and I was threatened and assaulted. The students who did these things were hailed as heroes by their peers. At one point, I identified who wrote me a threatening letter (I have studied graphology), and the student's parent wanted to see me. He wasn't concerned that his son had written me a death threat. Instead, he wanted to sue me to "clear his son's name."

In addition, teachers have been known to become frustrated to the point that they take it out on students.

So classrooms are very volatile situations as it is. Put a gun in every classroom, and what could possibly go wrong?