Now that I'm a little bit older, I realize why it was such a concern for my parents.
They were not worried about me smoking pot (which I don't btw, long story), they were worried that it'd make me lazy and jeopardize my future. They believe that it'll lead you into the wrong crowd of people. The drug itself is relatively harmless, your parents aren't stupid and know that.
The best thing you could do is sit down with your mom and dad. Talk to them like adults, look them in the eyes and talk calmly and confidently. Admit that you smoke pot occasionally. Make them understand it's under control and although you're able to stop at any given point in time, it's a social thing and frankly you enjoy it. If you approach it properly, they'll accept it eventually.
Then again, I don't know you or your parents. It could be they go apeshit, paddle your ass with a spoon, then send you off to boarding school. Luckily my parents always treated me as an equal and we were able to level on many things.
Keep in mind that odds are, your parents have smoked weed at some point in their lives. Likely, a lot of weed.