What should a female second-year high school student do if she had previously, both forgetfully and intentionally, delayed paying for some classes, and she does not remember the exact amount? Does she have to return the money, or is repentance to Allah sufficient?

If you did not pay the due fees for some lessons, you must repent and pay the money to its owners or seek their absolution. If they absolve you, then you owe nothing. If you cannot reach them or seek their absolution, then give that money as charity on their behalf. If you do not know the exact amount of money, then endeavor to give an amount that you predominantly believe covers the right.