“How should we discipline our 15-year-old when he gets poor grades? We had assumed that bad grades were enough of a consequence, but he doesn’t seem to care.”
I’ve yet to meet a kid who doesn’t care. The academic struggles children with ADHD experience aren’t caused by a lack of discipline or effort. Your son may be having trouble living up to expectations because he lacks the right strategies and supports; his frustration may have led him to believe he can’t do better, but I’m sure he’d like to.
Instead of looking for appropriate consequences, find ways to help him improve. Meet with each of your son’s teachers (individually, if possible). Get details about your son’s performance. Does he hand in assignments late or not at all? Fail to study? Not know what to study? Work with the teacher to solve each of his problems, assuring her that you will work with him at home.