Justin and his mom just had a non-conversation. Reading the drama reminded me of two people talking at each other rather than talking with each other.
If I were Justin, I would definitely feel attacked and immediately go on the defensive, which is just what Justin does. We can certainly understand that Mom is disappointed in his school achievement and wishes that he would apply himself more. But how can she get this message across to Justin without causing him to crawl into his armor to protect himself from the attack?
Another question that needs to be addressed is, how much control do parents have over their children? Mom would like to think that she can control Justin and force him to study, but can she?
It's important for Justin's mom to let him know that she has heard him, and while she may not agree at all, she understands his point of view. At least that way Justin will feel that he has been heard, and is not talking to the wall.
If Mom had said something like, "I hear you Justin, and I understand that studying is not high on your priorities. On the other hand, for your father and myself it is important. How can we bridge this difference?" What might come out of this is that neither Justin nor his mother will feel completely victorious, but neither of them will feel that they have complete lost. There is an opportunity for real communication and dialogue, without resorting to threats and punishments.
If Mom has not made any headway with this approach, and she has decided that she is the one holding the cards, she needs to find restrictions or punishments that Justin will care about. However, she should remember that punishments often boomerang, and while Justin may comply in the short run, in the long run she runs the risk of ruining her relationship with him.
Another thought is offering incentives to Justin for studying and improving his marks. The incentives have to be things that he cares about, and is willing to work for. Some parents find this option unattractive, as they would prefer that their children find satisfaction in studying and achievement.
The reality is that Justin is not unusual at all in his behavior. Many high-school students behave this way. While this behavior may be new for Justin and his parents, it may be an indication that he is trying to pull away a bit from his parents, and experience some independence.
His parents value education and achievement. He is trying out his wings to see how different values feel. If his mom will calm down a bit, there is a good chance that he will end up not too far from where he started. If she constantly batters and attacks him, he may adopt values that are far different from hers, and that she does not approve of.