It's normal and correct for educational institutes to withhold diplomas, degrees etc. when accounts are not fully settled. It's not correct to issue spurious invoices to extort money from students.

If the school were to go to a judge to demand payment for this invoice, would they win? If not, then that is one route you could pursue. If a judge decides that there isn't a debt, then the school will have to hand over the diploma.

Is the school part of some large organisation or group? Check what they have to say. They wouldn't want their good name sullied by an irresponsible member.

As you've not really been clear about the situation - who does the apartment belong to, why was your girlfriend living there without a contract etc. - it's kind of difficult to advise further.

If the bill is justified then that is the case. However if the bill is disputed you can get the non-payment removed, but you have to follow the procedure. Refusing to pay and writing letters is not the answer. There is a set procedure to follow and if you do and you can prove your point, you can get the record removed.