Dilemma of teacher-student ties

The Education Ministry, in its latest move, has ordered teachers not to sexually harass or have improper relations with students. Although overdue, this is a welcome directive. But an article in Beijing News raises a pertinent question: How to define the "proper relations"? Excerpts:

If a teacher, despite being married, has a relationship with a student, he or she should indeed be condemned, and even detained. But how does one judge a single, young or middle-aged teacher in a university or college who has a relationship with a student? Can he or she still be accused of having an improper relationship with a student?

Harvard University banned teachers from having "relationship with students in class" way back in 1984. After that, many countries ruled teacher-student relationships illegal irrespective of whether the teacher was a direct or indirect instructor.