My daughter canceled appointments at a driving school are we are still being charged, what can we do?

My daughter canceled two appointments with the driving school. She gave them enough notice and the school was able to fill the instructor's schedule for the day. The school sent us a bill and will not allow her to schedule more hours until payment has been made. They state that this is their policy and we were made aware of it during the parent orientation. However, when my daughter canceled the appointments they did not make her aware of the policy or let her know she was canceling outside their required time frame of 24 hours.