From what I remember, the main character was a son of a gang leader of some sort and had a tattoo of a black dragon on his body. And his love interest was a teacher (nursery teacher I think?).
There was also a situation within the manga where the main character was forced to sexually service his fathers previous lover, whose young daughter just so happened to attend the school of the love interest.

The manga you're looking for is Yondaime Ooyamato Tatsuyuki