From MangaHelpers & Kirei Cake:

It has been decided that Nishi High will close in two years time. Since then, the student population had dropped to a total of only seven. Hazakura Motoya, one of the remaining students, decides to stay for the free ride through high school.

Suddenly, everyone is shocked when they receive three new transfer students, Kayama, Mizumura, and Toujou, girls who are actually members of a special military division.

Soon after, Motoya and the three girls witness a giant tower suddenly appearing on school grounds. Motoya is chosen by destiny to enter the tower with the girls in order to uncover the mysteries of the tower.