Hey, thanks for your answers and nice to see others already having the same issue.
Yes, the only solution right now is to use the system as proforma invoice management.
But that way we have to handle each customer / student manually and write every invoice with our own system. This is really a lot of additional work and not so nice for the students.
It would be really nice when guru would have a usable tax system, so the invoices can be used in europe.