Answer

It depends what kind of exams. If it does not come at anyone else’s expense, then it is merely a lie. Lying is not formally forbidden by Jewish law, even though the Torah says, “Keep far from a false matter.” (There is an article by Rabbi Sherlo in one of the first issues of Tzohar, with a basic Shulchan Arukh-style treatment of the laws of lying.) There is certainly a moral prohibition here, by virtue of the falsehood itself. And perhaps it can be compared to the prohibition of deception, in Hullin 94a. If a person gets a high grade, he will receive high regard from those around him without any real basis for it. But in my view it is very doubtful that this is comparable, because this is more a matter of lying than of deception. I defined the difference between them in chapter 3 of my article in Tehumin:
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If it comes at someone else’s expense (whether another test-taker, or a future client who will need your knowledge, or the damaged reputation of the institution that examined you and gave you an inappropriate grade), then one must discuss the halakhic prohibition according to the particular situation.