Help!!

I read this a long time ago, but didn't bookmark it and now I can't go back and reread it.
The plot goes like this: a teacher noticed a student hanging around a yakuza and he came to the conclusion that this student is being bullied. And so one day, I think in a gym equipment room, the teacher found said student and the yakuza having sex and then it was revealed that this student were also the leader of a yakuza clan as well. The student also has a tattoo on his back, I think.
Any ideas? ^^