Valkyrie Bullet

Story

A Consummate Soldier Faces His Toughest Battle Yet: Attending an All-Girls School! In the near future, Earth has been invaded by monsters called giju from another world, forcing humanity into a never-ending war for survival. Conventional weapons are ineffective; the world’s only hope lies in the Valkyrie Dresses—suits of powered battle armor with extraordinary combat abilities that can only be piloted by women. But that all changes one day, when a man named Ren Hifumi is suddenly transferred to the Third Academy Valkyrie training facility.

Synopsis via AniList (English).