Classmate (♀) to Meikyuu no Futekisetsu na Kouryakuhou

It was decided that Nishi High School would close in two years time and since then, the student population had dramatically dropped down to a total of seven. The remaining students, including Hazakura Motoya, decided to stay for the free ride through high school. So everyone is shocked when they suddenly receive three new transfer students, all girls: Kayama, Mizumura and Toujou. But their appearance is just a cover; they are elite members of a special military division that reports only to the president.

Soon after, Motoya witnesses a giant tower suddenly appearing on the school grounds. According to his new classmates, the tower can only be seen by a handful of chosen people and that all the military technology that the country has been able to develop in recent years was based on the information gathered from inside the mysterious tower. Motoya has been chosen by destiny to enter the tower with Kayama, Mizumura and Toujou to uncover the mysteries of the tower.

(Source: MangaHelpers)