Of course, there are always exceptions, but for the most part, teenagers make poor instructors, especially of adults. I discuss the problem of awarding black belts to children and having children instructors in more detail in TKDTutor.com. We say that children are not responsible enough to be held accountable for their own actions legally, and yet we allow them to teaching fighting arts to other children. In general, the practice has lowered the credibility and integrity of the martial arts.
As to the student and instructor in your question, if the student had behaved inappropriately in the past, he should not have been awarded a black belt to begin with, and he certainly should have not been allowed to test this time. Of course, this means that the school would lose revenue.
I am from the old school of martial arts training, which means I learned to punch and kick with full-power and full-extension, but while using precise control and focus. I punch and kick, very hard but I do not hit the opponent, however, if the opponent blocks, he may hurt himself. In tournaments, I have been warned many, many times about excessive force even though I do not hit the person. When the opponent blocks an attack, the impact makes a lot of noise and sometimes knocks him down so I am warned even though it is not my fault. The opponent does not have to block. If he does not block at all, he may lose the match, but he will not get hurt.