It was wise to withhold your email address as you're likely to cop many irrational, denigrating responses.
Firstly, whatever you decide to do, be very mindful of the long-term damage that will occur to the student's reputation / status if your relationship becomes public - now or when she becomes your student.
Chinese university classmates tend to stay in contact for many years after graduating and call upon each other for favors from time-to-time. Your girlfriend may permanently lose both mianzi (face) and guangxi (relationship).
Given that you are already in a relationship with the student, I'd imagine that things would be much more awkward in the classroom if you end the relationship now, than if you preserved the relationship.
If you want to end the relationship now then you really need to leave the university now.
If you choose to preserve the relationship, stress to your girlfriend the need for extra vigilance in maintaining secrecy and explain to her that you will have to treat her equally, if not indifferently, when she is in your class.
Only you can judge if she is mature enough to handle the changed circumstances.
I am a 37 year old FT at a large university in Hunan province. I have been in a relationship with a twenty year old student at the same uni for six months. We get on really well, have many common interests, and have a satisfying sex life.
However, to my dismay, I have discovered that next semester this girl will be one of my students. I don't know what to do about this, it will be strange having my GF in my classes. I have kept this relationship very discreet, as my apartment is on campus, and my GF only comes to my on-campus apartment three nights a week, after 8 at night when it is dark. She returns to her dorm by 11pm at night.
So far nobody is suspicious of us, but I am worried that someone will pick up on the vibes and chemistry between us. I also don't want to be called a "cradlesnatcher" by anyone if they discover our relationship. It is a loving relationship we are in.
Should I split up with her? Any advice would be greatly appreciated.