If you know they're going to disapprove of it, you'll just regret telling them. Keep it a secret if you think you can reasonably get away with it. Not everyone gets caught; you only get caught when you're not careful enough (trust me, I've been there). Do everything you think is necessary to ensure that you don't get caught. Smoke outside, smoke at night, smoke through a spoof, smoke out the window, whatever. Just don't smoke when there's any possibility whatsoever of your parents finding out that you're smoking, and make sure you don't act obviously stoned around them afterwards, and you should be okay.