I had a philosophy teacher in college that I used to burn with. Pretty neat fella'. But, he was the one who brought up the ganj, not me. If you know he smokes, then chances are, he knows you smoke. If he wants to burn one with you despite the consequences, then he will probably bring it up one way or another. Chances are very good though, that even if he does smoke, he wouldn't smoke with a student. It's just a degree of keeping your personal life and your private life seperate.