If you catch your teen and his friends smoking pot, first, don’t panic. You’ll handle the situation most effectively if you can remain calm. Then, says Phillippe Cunningham, a Ph.D. in clinical psycholog and assistant professor at the Department of Psychiatry and Behavioral Sciences Medical University of South Carolina:

• Be very specific and clear with your son that you do not want him to use drugs or associate with peers who use drugs.
• Punish your son's association with these drug-using peers by taking away his privileges contingently (examples can be found below) and positively reward association with peers who you do not believe are using drugs or alcohol (i.e., access to privileges).
• Contact the parents of his friends and let them know that you found your son and his friends (their son) smoking pot in your house and as punishment you are limiting your son's contact with their son (a minimum of one week, however, if you catch them again you may have to mandate no contact at all). Association with drug using peers is the number one predictor of adolescent drug use.
• Contacting the parents of your son's friends can be helpful in several ways: (1) it sends a clear message to your son's friends and their parents that you are serious about your son not using drugs, (2) it often leads to other parents punishing their children, (3) it provides a useful model for other parents on what to do if they were in a similar bind, (4) it elicits help from others in keeping an eye on your son (it does "Take a Village to Raise a Child"), and (5) it may serve to embarrass your son, which is a powerful motivator for most adolescents.
• If you have a close bond with your teen, it can be very powerful to make him understand that he has to regain your trust through mandatory supervision. In this approach you would not allow your son to be home alone without adult supervision (which may inconvenience him) and requiring an early curfew. However, as your teen shows you he’s doing what he’s supposed ot do again – from homework to chores -- you can begin extending curfew and increasing the amount of time he is without "close" adult supervision (but you must continue to check on his whereabouts and peer associations).
• Pay attention to your son's efforts to act responsibly. Because exclusive use of punishment has the untoward effect of undermining the emotional bond (not to mention increased anger, resentment, and "sneakiness") you must set up opportunities to catch your son being good.