Does Klasku remind parents who have not paid?

Yes, automatically. Reminders are scheduled at day offsets around the due date that the school sets itself — 7 days before, 3 days before, on the day, 3 days after, and 7 days after by default — and one invoice gets one reminder per offset, however often the check runs.

Beyond that, staff can send bulk reminders from the invoice screen, or forward a single invoice over their own WhatsApp from its detail screen. Invoices past their due date are flagged Overdue, and the number on the navigation badge uses the same predicate as the screen it opens.