From Kirei Cake:

It's been declared that Nishi High will close in two years. Since then, the student population has dropped to a mere 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 the school grounds. Motoya is destined to enter with the girls in order to uncover the mysteries of this tower.

Digital publication: http://seiga.nicovideo.jp/comic/40782 and https://comic-walker.com/contents/detail/KDCW_AM05200900010000_68