First, I agree with robert42 - Chill! Serrously!

Second, if you want to tell them, lay it all out. Explain that you've been smoking for a few months and show them that it has had no negative effects on your schoolwork. Perhaps an agreement that if your marks start slipping you would stop smoking would be a good gesture.

Ask your mom to explain to you exactly why she thinks it's a bad idea for someone your age to be smoking. If you understand the reasons behind her take on things, you'll be in a better position to soothe her worries and possibly make agreements that will keep your parents happy and you tokin'.