I know what you mean, zolly. I'm in that phase right now.
I think it was that movie, Office Space that changed me. It was the phrase "I'm not lazy, I just don't care!" that became my motto. But after getting some bad grades, I've decided that it's worth it to trade time on the computer, with time doing homework.
In fact, today I had a parent-teacher conference and they said my grades were fine the first trimester, but they sort of did a backslide in the 2nd. One teacher (which I have a B in his class) said it wasn't a lack of interest, I just was too tired to do anything. Personally, I think it's a lack of interest. And I have a B in his class, so he can't say anything unless my grades are low.
Another teacher said I just wasn't organized enough, which I think is half the battle, for me anyways.
I picked up a hobby, drums. I stopped playing basketball and doing karate as much, but I'm really advancing with druming and I like it a lot. Karate stopped being fun for me, and I already made it up to black belt- I lost interest in basketball.
My motivation for my grades? A sense of fullfillment and...summer school.