A lot of things happen in adolescence. Many of their patterns become set in place. They are trying out new techniques and values and what will be successful. That is why they are so inconsistent. The parent needs to look to see what kind of things pushes the child's buttons and what the child responds to.
Parents have to listen to the child.
Sometimes parents don't like the message that the child is giving them so they don't listen. Listen to the unsaid words or implications. If you can't figure out where to go with a child, then have them assessed. Students so often sell themselves short. A child that feels he can't do something won't risk himself because he is afraid he will fail.