My general rules for student teacher dating (in or out of China without respect to nationality or gender of anyone involved, but assuming all parties are of legal age) would be:
1. If there's a grade involved and the student is enrolled or is planning to enroll in that teacher's class, then no. A person who controls a grade has power over a person being graded. If there's some true mutual attraction, then let it wait until later. If it's true love, it can wait until after graduation (see #3). If it's only hormones, both parties likely just saved themselves some awkward moments.
2. A teacher and a student in another department who will not ever take a class from that teacher (grades still involved). - Possible, but highly risky. Other students could presume that dating any instructor gives some form of preferential treatment. May be against school policy (official or unofficial). If significant age/maturity gap (in favor of teacher), may easily result in student's relatives approaching teacher from behind with sharp objects and an intent to cause (possibly justified) bodily harm.
3. Teacher and former student (graduated). May raise some eyebrows. People will wonder exactly when the dating began. Could be used against the teacher during evaluations. If the teacher does this with multiple former students, the likelihood of this increases. Depending on who the former student is connected with, a bad breakup could have more fallout than expected. Then again, if it's really true love, what's a few slings and arrows?
4. Adult Education (no grades involved). Somewhat risky. May be against policy (official or unofficial). May result in resentment of other students. In the event of an unpleasant breakup, could cause some fallout for teacher's job (then again, dating a student's relatives in a case like this could also have fallout if there's a bad breakup, but no one ever said you couldn't date an adult's siblings, aunts/uncles, etc., but the fallout from a failed relationship can be similar to a failed relationship with the student - in a small town, dating any local person could have this effect). Assuming there is no policy against it, may be approached, but with caution. Keep in mind that the student's reputation may be tarnished if the relationship fails (in some cases, the teacher's reputation may also be tarnished - depends on the circumstances). If you chose to swim in these waters, do so with your eyes open and at your own risk.
5. High school. Even if the student is 18+ years old and not in your class, a very bad idea.
6. Dog obedience school. Illegal in most countries.