Hitoyo Hitoyo ni Otomegoro

During a military experiment trying to produce a super-soldier, Kazuma Kurosaki was accidentally transformed into a girl. Until his feminization could be reversed, his superior officer, Lieutenant Sakurakoji was assigned to look after him... Kazuma ended up attending an all-girls school to act as a bodyguard for the Lieutenant's younger sister, Hyuko! A gender-bending love comedy!

(Source: Shounen Gahousha, translated)