Should teachers be allowed to carry guns at school?

In terms of how it actually is, that depends on where you are. Some places allow it, some do not. For instance, in Minnesota you can carry in a school if you have a written authorization from the school's principal in addition to your carry permit.

In terms of whether it should be allowed, there is no logical reason to forbid it. If a person is deemed to be capable of carrying a gun in public at all, it really does not matter whether he is in a school, church, shopping mall, or just walking down the street.