Rainbow

From: Easy Going Scans
Six minors (aged 16-17), known by the nicknames Joe, Mario, Suppon, Baremoto, Heitai, and Kyabetsu, find themselves sent to a correctional facility for offences such as aggravated assault, swindling, and fleeing the law.
Their stay in hell begins immediately. Accommodated by a doctor pedophile, they are placed under the watch of a despotic and brutal crew. Cellmates with a certain Sakuragi, also known as Anchan, the tension rises between the band of buddies and their new "friend."
A brawl breaks out, and our six prisoners are done rossés by the mysterious young man, thanks to his boxing talent. The story not only explores the lives of these seven characters as they struggle to survive in their hellish environment but also delves into their disillusionment caused by rejection from the outside world and their close relationships. This is a tale of an extraordinary and unbreakable friendship among these young men.

Note: Won the 2005 Shogakukan Manga Award for the seinen/general category along with A Spirit of the Sun.