If he is not pressing it on any students I don't see the harm. I had a english teacher that had the 10 commandments on the board and I never seen anything wrong with it. he would NEVER EVER referance them. they were just written in chalk and he would erase and rewrite them everyday.
BUT. I do not think that public school is any place were religion should be taught. You can't get 2 christians of the same sec to agree how are you supposed to get everyone else to?