Is it illegal to be blackmailed by your student?

Is it illegal to be blackmailed by your student? summary:

Tenjin, a 27-year-old teacher who works at a cram school for students taking entrance exams for high school and middle school. While he is talented at teaching elementary school students, he holds no love for children. He coldly contents himself with grinding through his job while training his colleague JD and being tossed about by the cruel currents of life. But one day, while teaching a fifth-grade class, one of his students hugs him close and tells him that she loves him. And a 14-year old middle-school student named Seika sees the entire thing. Seika blackmails him into giving her "personal lessons" at night.