How is this different from keeping a spreadsheet?

A spreadsheet records; Klasku executes. Attendance marked by a teacher deducts the student's remaining sessions right then, invoices are issued on the date you set, reminders leave on their own, and parents check their child's remaining sessions without asking the admin.

The difference shows most when a number is disputed. In a spreadsheet, remaining sessions is a figure somebody typed; in Klasku it is the result of the attendance history, so it can always be traced back to the session that deducted it — including when that attendance is corrected.