My invoices must include VAT — can it be calculated automatically?

Yes. The tax rate is set once in the school's settings, and every invoice issued afterward computes it itself: tax on the subtotal after discounts, added to the total, with its own line on the invoice the parent sees.

The rate in force is frozen onto the invoice as it is issued. Changing the rate next month therefore does not rewrite old invoices — the archive keeps explaining the amount that was actually charged at the time.