A "discreet" sex would be masturbation in the loneliness of the North Pole.

Discrete sex, on the contrary, is something else. All sex should be discrete, unless in a voyeuristic/exhibitionist club.

And, of course, sex with one's own student is NEVER acceptable. There are millions of people in every Chinese city. One should even refrain from sex from other students at the same school whom one does not teach, but one's own students -- ABSOLUTELY NEVER.