Rather than registering for a specific course and being constrained to only take that course and paying for classes you miss we use either a punch-card approach where you pay for just the classes you attend or a monthly fee that allows you to attend as many classes as you wish.
With a punch card you purchase a pass allowing you to attend a fixed number of classes. You only pay for the classes you attend in whatever program you choose.
With a monthly membership you pay for a specific number of classes per week you wish to attend (unlimited, 3 per week or 2 per week). The restrictions are the monthly memberships are non-refundable, not transferable to other people, non-transferable from month to month and all missed classes must be made up within the payment period.