Avoid looking down on people who swear, watch scary movies, dress immodestly, or do more "adult" activities than you do. Work hard in school, raise your hand, and get the best grades you can. This just makes them so much more interesting.
Look for the positive side. Express your feelings in a straightforward, non-accusatory way. My Babysitters Club. If someone tries to get you to cheat, say "I'm not comfortable with that.