As-salāmu ʿalaykum. What is the ruling on a student studying at a mixed university who feels unable to leave because, if they were to ask their parents to withdraw, they would likely become upset or angry, and insist that the student continue, especially since they are paying the tuition fees?

We should worry about upsetting Allah the most. There is always a way out and there is always an alternative. Look for a solution for it rather than just dropping out and ruining everything. Maybe you can find an alternative, like an online degree or another university, without losing out on the progress that you have done.