In your email to parents notifying them of the auto debit that will take place, please make it clear to the parents that past due balances will not be auto drafted on the 1st of the month and that the parents are required to make manual payments for past due balances.
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 ...