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Gunnou Kyoushitsu

All of his classmates, including his best friend and his crush, have been reduced to nothing more than brains. An alien who calls herself A-sensei informs the protagonist, Satoki Tsutsuno, that to protect the earth, they must use their collective brains. With Tsutsuno being the only one left in his class, what trials await him and his classmates...? A shocking and suspenseful story of collective brain warfare.

(Source: MANGA Plus)