Fortunately, there are many things parents can do, not just for teen-agers but for the whole family as well. In general, what to keep in mind first is that parents and teens have very different notions about the purpose of school.
School is their job.
Therefore, they should get good grades, just as we want to do well at our jobs. The teen-agers' perception is quite different. In their view, school is rarely more than their social world, interrupted by six or seven classes a day. While these are activities to have started from Day One with your child, you can still implement them in your home now. In whatever way works for you, make sure your teens know that while grades are vitally important, your children are more important to you than their grades.