Is there a reason why you don't allow for switching?
I have a part time family, who's schedule is changing constantly due to hospital hours and hubby working handy man jobs in between classes at college. I don't mind it at all, and if I can accommodate it, great. They must attend a minimum of 3 days per week, so as long as they hit that I'm fine with it. Just trying to grasp why it bothers you so much?