I never had much motivation for school anyway. Every subject we have is, in my opinion, utterly and completely useless to me. More so in math.
I usually get average grades at C's or B's. This semester I got a B, two C's, and 3 F's. 2 F's were in math, the other were in US History. Not only do I despise that class, but we had a student teacher this six weeks. She couldn't teach for shit. All we did were maps and videos, and I can't learn like that. I have to read it. So, I gave up about 2 weeks through.

But, like Buck268 said, as long as you do your homework and pay attention, you'll know what the hell is going on for test time, and your grades will be fine.

I got 1600 on my PSATs, and the guidance councelor said I'm probably gonna get damn near perfect on the SATs. All my teachers get fed up (they all really like me though) with me sometimes because they're all convinced I'm a genius, but I have frequent "it's not my day" days, in which I come into class and don't pay attention or don't do my work. Which is true.

Weed hasn't hurt my grades, I've always been this way, and as long as I do good on my SATs, I'm not worried.