The bank details must be stored so that the QR-bill Switzerland can be activated.
Go to Extras > Settings > Skischool:
Then you just need to activate the QR-bill.
To do this, go to Invoices > Extras > Customise > QR-Rechnung (CH) > Check "Enabled"
When an invoice is finalized and printed, a separate PDF document is created with the payment slip. It is currently not possible to display the QR code directly on the invoice.