From Volokh:

What Exactly Is the Reason Not To Allow Professors To Carry Guns?

We’ll, let’s see.

In all my encounters with nutty students (there have been a few), not having one has never been a problem. Walking onto a college or university campus, I am in one of the safest environments in America.

Besides, no one should feel that there is a physical manifestation of whatever repression he or she may believe is affecting them.