A Turtle's Shell is a Human's Ribs Kame no Koura wa Abarabone

Kameda (turtle-human), a normal student at a high school with other animal-humans, leads a dull life. Although he’s in the least popular clique, due to a chance happening one day, he decides to open his “ribs” (mind) and face it head-on. This is a tale of conflict and growth of these animal-humans.

(Source: TypeZERO)