Quote Originally Posted by Just1n
Dude, I was in a very similar situation in high school. My parents discovered that I use cannabis regularly and were pretty disappointed by it. You wanna know how I eventually got them to shut the fuck up? WELL!? DO YOU?!

I graduated highscool with a 3.5+ GPA and went on to a challenging private university. Considering that neither of my nuclear parents have a college degree, they couldn't say shit about me being a 'lazy stoner'. And I smoke EVERYDAY, at least once.

The fact that you have a low GPA and show no sings of being successfull is a legitimate reason for concern for your mother. I realize that she probably has a warped perception of MJ and that is unfortunate. But it doesn't help the situation when you do nothing to proactively change her views on pot. Do us all a favor:

Get a job, join a club, fucking study for all I care. But do SOMETHING positive to show her that you can be productive AND be high as fuck all day. If you can't do that, then hang yourself with a piano string because your slacking is giving us all a bad name.
Very true. Actions speak louder than words. Continue to smoke if you want, keep it secret from your parents, and get your shit together. Get a job and get As. I realize how meaningless high school grades are, but trust me, it will help. The best way to prove you're not a lazy stoner is to not to be one.

Other than that, sit her down and talk to her. Ask her why she doesn't want you to smoke, and refute every negative thing she says. May want to read up on weed a bit. Erowid is a nice source. Bring up the positives as well.

Good luck.