Am I sinful for not remembering whether I returned the fine money I collected from students for use in the classroom, which was nine years ago, knowing that I have donated many times its amount with the intention for it to be for them?

The money taken from the female students without their consent, as a punishment, must be returned because it was taken unjustly. The liability rests upon the one who directly took it. The money must be returned to its rightful owners or their absolution sought. If they absolve the person, then there is no harm. Otherwise, it must be returned to them if possible. If not, then it should be given as charity on their behalf, while informing them and giving them the choice to either approve the charity or take their right when they are reached. If the exact amount of the money is unknown, then what is certainly the amount must be given.