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.