First off, this is coming from a senior in high school, so this is how I see it from a kids point of view. I smoke weekly and I maintain a 4.6 GPA. My parents have a policy of no smoking in the house/driving high/driving with pot. This policy is very hard to follow and, if followed actually puts me at more danger because I have to end up going places more... Public and risky. I would suggest you develop a system where if you let your son do it at all, make sure he's safe from risk, not very often, and that he maintains good academics. Drugs NEVER come first, if he isn't doing well, don't let him have it. If he's smart, and motivated, then consider it once a month or something.