Nanbaka

The series focuses on Jyugo, Uno, Nico, and Rock, four young prisoners at a unique prison where inmates are divided into groups and assigned numbers. They comprise group 13 and, despite the circumstances, take every opportunity to have fun, much to the chagrin of their guard Hajime. However, Jyugo's leisurely life is upset when he learns there is more to his past than he remembers...

(Source: LiveChart)