AFAIK in many countries and universities around the world this is no problem in 90% of the cases. Good professors are more than happy to have you there. One I

AFAIK in many countries and universities around the world this is no problem in 90% of the cases. Good professors are more than happy to have you there. One I even asked to take the exam, and get it corrected (to test if I really learned the content). And they allowed me to do that, I got the corrected exam at the end. Of course, it was not written in any book in the university.