Seisen Kyoushitsu

Makoto Uda, a high school student, and his childhood friend Yuki Mika have a secret. The two of them lived a peaceful high school life while hiding their secret. Until that day, when the high school that Uda and his friends attended was divided into "good" and "evil" by those guys... The classroom was transformed into a bloody battlefield where students killed each other.

(Source: Kadokawa, translated)