Otokogumi

(from ebookjapan): A boy, Jiro Ryuzen, is imprisoned in Kanto Juvenile Prison for the crime of "killing his father. He was sent to Seiun Gakuen, a prestigious private school, in handcuffs, which was ruled by the immensely powerful Goji Shinryu... Together with five members of the Gokehoren, his allies in the juvenile prison, Nagare's fierce battle has begun!