Allow parents to schedule make ups

It would be great if we could allow parents to schedule their own make up classes. To do this, we would need to specify which classes would count as a make up for other classes. i.e. Ballet 2 can be made up in Ballet 1 or Pre-Ballet. Then the system would need to allow make ups to be scheduled based on the enrollment and trials for a class so we don't get make ups scheduled for a class that's full.