It would be great if you were able to block out certain dates so clients can't book make up classes. I receive high volumes of permanent bookings even once term has begun, usually within the first two weeks of the term. Clients who have booked a make during these dates have to be rescheduled because permanent bookings take precedent.
We've added the ability to block Make-up Lessons being booked through the Customer Portal for a specific date range. This feature was added to SimplySwim in release 56.
Go to Configuration -> Customer Portal -> Block Make-up Booking Date Ranges to add your date blocks.