the key thing here is respect and intelligence. have a real talk with your parents about it, not a teenage bitchy "your ruining my life!!!!" argument.
explain that you know why weed isn't very bad for you. explain that it hasn't changed any part of the son that they are otherwise proud of. explain that weed is not even close to as important to you as biking and skiing, thats most important. explain that if you can't bike or ski, what the fuck can you do? at this point parental punishment is screwing up your life wayyy more than weed ever could.
now your not gonna convince them to let you smoke or anything, but you will show them that you got a head on your shoulders. they want to be reassured that your responsible, and if you can have a serious discussion with them about it that will help. oh yea and getting good grades always helps too.