What is the ruling on studying at a mixed-gender university if it is the only one available, and the parents refuse to let their child leave it after repentance?

One must obey their parents unless they command disobedience. If your continued presence at the university leads to sins that cannot be avoided, then you must leave the university, even if your parents are displeased, because "there is no obedience to a created being in disobedience to Allah." However, if you can avoid sins, adhere to Islamic manners, and refrain from intermingling, then obedience to your parents is obligatory, and you can combine studying with obeying them without falling into sin. Be keen on inviting to good and avoid addressing women.