My 13-year-old son has been on the seventh-grade honor roll all year. His last progress report showed a dramatic drop due mainly to not completing homework. I ask him every night about homework and he has been lying to me. I have informed him (and his teachers) that if he doesn't turn work in, or turns it in late, he will have to miss the end-of-the-year amusement park field trip. When I ask why he is doing this, I just get "I don't know." Any advice?
Good for you -- you have set a boundary and an expectation, and following through will help your son to understand his responsibilities for completing homework.
May I make a suggestion for the future: Set up a nightly study time and require that your son spend that time with no music or TV. He can quietly read if he has no homework. It is amazing how much homework gets done when an adolescent has to stay there, anyway. This also makes it very difficult for him to say "I don't know." What that usually means is "I don't want to tell you" (that I enjoy all my free time hanging out or talking with my friends).
Keep setting those natural and logical consequences. You and your son will be much happier.