Get my teenage son to turn in his homework!

What do you do when nothing you try motivates a child to turn in their homework? My son does well on all of his tests, but turns in very little homework. As a result, he has failed most of his classes for the 2nd quarter in a row. Even when I make sure the assignments are done and in his backpack, they still do not seem to make it into the teacher’s hands. It got to the point this quarter where he was grounded from everything: tv, computer, cell phone, going out with friends, etc. He cannot get his license until he passes in school. Nothing I do seems to motivate him.