Try doing some family stuff on a regular basis. Do something your child likes or is interested in. Try to keep him or her busy with activities. Laugh a little. Laugh a lot with your child.
Laughter does wonders in healing depression and anger.
Don't push too hard on the school thing until the treatment program with the therapist has been started and is working. A depressed person isn't going to be thinking clearly enough to do anything well, and may have spells of forgetfulness. If the child has been doing drugs, the treatment program will have to be working and the drugs out of the child's system before the child will be able to function at full capacity again. Encourage your child, but don't make a federal case out of the grades right now. Your child's life is at stake.