It is a mortal sin.
Uhm, no; not necessarily.
There are many things that could change the nature of the sin. Did someone just cheat on one question and raise the score from a 90 to a 95? What is the importance of the exam and the consequences of cheating? Was someone else defrauded of something based on the cheating?
It depends on the consequences and the circumstances. So, for instance, if someone cheated on their medical boards and became certified as a doctor only as a result of cheating, that would likely constitute a grave sin. If cheating allowed you to get a scholarship you didn't otherwise deserve. There are lots of factors involved. Just ask a priest and explain all the circumstances. There's also the subjective disposition up front. Did the person think it was a mortal sin before or during the act and will to do it anyway? It's not easy and should not be answered definitively by the untrained members of an internet forum.
Why would the consequences affect how serious a sin it is? Surely it should be the person's intended consequences. For example, we can do good deeds and accidentally cause bad consequences. For example, a man in WW1 spared Hitler. Had he cruelly executed the wounded Hitler then, he may have prevented WW2 and saved millions of lives. But there was no way he could have known that, and it certainly was not a sin. His act was an act of mercy and charity, and executing Hitler would've been a mortal sin, even if the latter might've had better consequences for the world.
Similarly, if one cheats *thinking* they will rob someone else of a place, it is a serious sin whether they actually do or not. And if they do not think their cheating will adversely harm anyone else, it is a far less serious sin, even if they do end up accidentally adversely affecting someone else. The act is the same, and it is a sin either way, but it is the intent that makes one a more serious sin than the other. Not the unintended consequences.