I am assuming you are a senior and have already been accepted to a university. In that case, the only way your exams will make a difference is if (1) you were eligible for a scholarship based on your grades, and (2) if your exam grades lower your overall GPA significantly.
Since the university accepted you based on your grades, test scores, and activities, the university can rescind your admission if your grades drop too much.
Most universities send out a conditional acceptance letter and make their offer final after you have actually graduated high school.
