Discuss your teenager's study habits with him. Ask him how he likes to study and why, what he feels his strengths and weaknesses are and similar questions. Explain that you want him to use his study time effectively so that he will have time left over for activities he enjoys more.
Show your teen how to be an active textbook reader. Emphasize that she needs to spot topic sentences and supporting points to understand the main points of chapters.
Teach your teenager organizational skills. Make sure that his binder is divided among all the classes and that each class folder contains the proper notes. Look at his backpack and make sure there aren't crumpled papers in the bottom. If there are, ask where they go.
Make a checklist of school subjects and extracurricular activities for your teen. Every day, go through and ask him what is due in the upcoming days in each subject and when tests are scheduled. You might have to hammer him over the head with it for a while, but eventually he will start keeping track of this information on his own.
Help your teenager study for tests. Show him how to use flash cards and outlines. Make it clear that he can only study with friends once he improves his performance.