Step 1: Thoroughly review your teenager's last report card and make notes on it.
Step 2: Let your teenager know that for every grade that is below your expectations of his/her success, he/she will have to spend an additional amount of time per day studying for that subject.
Step 3: Create a three-column form: Subject, Assignment and Teacher's Signature.
Step 4: Have your teenager write out assignments and homework on that form. Make sure he/she has work for every subject every day, even if his/her teachers do not assign it. Use exercises at the back of each chapter, or online resources to give your teenager additional assignments.
Step 5: Go over your teenager's work once he/she has finished. If the work is not properly written out, has errors, or does not meet your expectations for whatever reason, have him/her do the work over and explain his/her errors. Contact the teacher if you cannot understand a topic.
Step 6: At the end of the week, make sure your teenager has obtained a signature on the three-column form from every teacher. The teachers might also write down notes or issues about your teenager that you were not aware of. They see your teenager interact with others and perform in school, so they might be able to give you important information that can help you in understanding your child's bad grades.
Step 7: Review your teenager's report cards for every term to see if grades are improving.