I'm done being their friend and it's time I start being their boss as well. I just e-mailed them all and told them there has been a change. I made the schedule in advance and told them to choose from which pre-set class times they want, rather than making me try to work class times around their schedules.
Operation crack down has begun. I'm going to start being like other employers now. I set the schedule now, rather than trying to make everyone happy by working around their other jobs and college schedules (I do still take their schedules into consideration though when planning ours in advance). And while their contracts do permit them to take a couple sessions off if they want to or need to, they will no longer be allowed to take that time off unless they find another staff member to agree to cover for them. Of course it's a little late to enforce that one since most of them have already made vacation plans for my busiest season. But next summer's busy season will be enforced in advance.