Request for Ideas: Student Payments.

I would like some way to represent whether or not a student has paid for a particular period of practice.  For example, if your school does monthly payments, then how can I make a generic transaction be good for March, 2009.  Not every school does it monthly.  Some schools have contracts for longer periods.  Some schools do mat fees.  I thought about adding a Date Range for "Effective Date" but that wouldn't really apply to transactions that weren't associated with students.  I guess the date range could mean anything you want it to.  Thoughts??