I had one of those too.....

Try not to get too pissed off about it, she is doing what she "thinks" is best for you and honey that is a damn site better than having parents that don't give a shit at all about ya. She bugs you because she loves you and as paranoid as she is about it, I bet she knew someone who's life got fucked up because of drugs. She loves you, just try to keep it out of her site and stuff.
When she comes up with her paranoid "I know you're high" don't get defensive, get rational with her.
How would I get high at school?
Do you really think I would risk taking drugs at school?
I respect you too much to ride around with you right after I blaze mom.
That lighter isn't mine, you don't have to beleive me but please don't punish me because you think it might be.
And above all, never, hardly ever, lie to your parent's, that trust thing is easy to break and fucking hard to get back.
This kind of stuff might lead to a real discussion, and even if it doesn't she will know you better and if she see's you making some semi-responsible decisions she may lighten up on you.