Hi, Kate. My son is in 8th grade, too, and his work habits have changed. He doesn't seem to care about getting homework done like he used to. I too am struggling with the best way to address this issue. What makes it difficult is that he feels it's his business, and that he's old enough to handle this on his own. And, to an extent, I agree with him! More and more often, he will have to live with the consequences of his own actions and I don't want to interfere with his process of experiencing natural consequences. I have decided to deal with it this way - I set limits ("All homework must be attempted. A zero for not turning in an assignment is not acceptable.") He doesn't want to be nagged about his work, and I need to respect that. But, when the report card comes, if it becomes clear that homework hasn't been handed in, he is aware that there will be a consequence and of what that consequence is. Anyway, it's all whole new ballgame parenting a teenager. Knowing when to hold on and when to let go is not always easy. There are no cut and dried rules that work.
Good luck. It's nice to meet another parent of a teenager!