Yeah, same kinda stuff happened with me. It took my mom a couple months to come to terms with it. Say that you only smoke occasionally..every other weekend or something..like as a treat. Be honest (minus the frequency of smoking), and they'll usually be less harsh. I'm sort of glad that my mom found out, since she doesn't care as much if my grades are up, and I'm not getting in trouble. It saves that butterfly feeling every time she calls me in her room to talk to me (usually about classwork). Anyway, good luck with that, and hopefully your mom won't be pissed for long.

My friend got caught, and is still taking drug tests every couple days about 3 or 4 months later. But then, he was caught with Acid, and Molly (pure mdma), and then when he had to talk to a counselor, she told him everything would be confidential, so he told her everything he had done (a lot of stuff..he'll try anything mostly), and they ended up telling his parents.

My advice: Tell 'em you stick to the herb (dunno if you actually do or don't), and tell them it's not that frequent. If your grades are decent, they shouldn't have much to say.