All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His slave and Messenger.
According to what we have understood from the question, you were receiving money from the school for a period of time that you did not attend your classes in the first place and you did not deserve getting that money; accordingly, you wish to clear yourself of your liability by returning that money to the school. If this is the case, then you can estimate the amount of money that you have taken without right and give it back. If you received one thousand dollars, for instance, then you should return the one thousand dollars and nothing more, even if the usurious interests amount to one thousand or more; you should return only the original sum for which you are liable. You would clear yourself of your liability by returning the exact sum of money that you received from the school and do not have to pay any interests. The money that you will pay back covers only the money for which you are liable.
Allaah knows best.