In My School Books every family account carries a balance (either negative or positive). If the balance is negative, the family owes you money. If the balance is positive, you owe the family lessons.
If a family misses a payment, their balance automatically carries over to the next invoice. So even if you don’t notice the missed payment, the next time you create invoices the total amount you’re owed will be shown on the invoice. This means that you don’t need to explicitly track which invoices are paid. Just record any payments you receive into My School Books and we’ll take care of the rest.