The boy, a second-year student at a state junior high school in Tokyo, allegedly forced his girlfriend, a third-year student at the same school, to sell her body by assaulting her and threatening to leave her. The money went to pay off his gambling debts, police said.
Since the two started dating in June the girl has allegedly prostituted herself 10 times.
The police said the boy admitted to forcing his girlfriend to have sex with a 39-year-old teacher for 30,000 yen (Pounds 154) in violation of the Child Welfare Law.
The rendezvous is alleged to have been arranged through a "telephone club". These clubs, common in Japan, are used by adults to buy girls as young as 13.
Tokyo has only just banned sex with minors to deal with the rising numbers of girls prostituting themselves to older men. The activity is known as enjo kosai or "compensated dating".