Rainbow

From: Easy Going Scans
Six minors (16 - 17 years) answering to the nicknames of Joe, Mario, Suppon, Baremoto, Heitai, and Kyabetsu are put into a reformatory for offenses such as aggravated assault, swindling, flight, etc.
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 (called 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 will not only follow the lives of these seven characters surviving in their hellish environment, but also their disillusions due to the rejection by the outside world and by their close relations. This is about the extraodinary and invincible friendships between these young men.

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