Some parents are not aware of your practice not to auto draft past due balances, and they have assumed that all outstanding amounts (past and present) are auto debited on the 1st of the month. Hence they are not happy when they find out that they have been charged a late fee because the past due balance was not auto debited.
This would be a great idea. Furthermore, simply functioning like an actual payment collection system and automatically collecting past due balances would be AMAZING. Then, we would get about 10% of the value of the cost of the terrible system.