I have a separate rate for my teachers. It is higher than regular clients and care is only available to them September through June, and not over christmas or march break. Tuition is due September through June, based on enrollment, not attendance. Basically, I've taken what a full time spot would be year round, and then applied that directly to the available days of attendance. So if regular rate is $35, then a teacher rate would be $40 (or thereabouts).
Then if they want a spot for September, they pay a deposit in June to guarantee their place, otherwise they take their chances it will be gone.
And I do not take their kids over the summer.
I like your business strategy nschildcare! I think I may adopt it! Currently I offer a 3 day week for summer breaks. March Break/Christmas/etc they pay full fees.