Is it permissible for a female student to intentionally fail her high school final exams so as not to take the place of another student, and what is the ruling on the gifts she will receive if she is among the top honored students?

So long as the matter occurred with the consent of the official responsible for selecting the top students, there is no blame upon you for it, because the mentioned condition is subject to the responsible authority. In addition, there is your excuse for not sitting for the previous exam. If they waive it, there is no problem, and by that, you would not be unjust or transgressing the right of another.