Tokoshie x Bullet: Kaijuu Gakuen

Ten years have passed since humanity suffered an unprecedented attack by a giant biological weapon controlled by a terrorist group calling itself the Tokoshie. Tsukimi is a high school loner. When a somewhat spacey boy transfers to his school, he appoints Tsukimi as his "partner," and his boring school life drastically changes into one in which "the survival of the human race" is at stake!

(Source: Afternoon Magazine, translated)