From MangaHelpers:

The man-made island of Nanaejima houses a population mostly consisting of high school students. The island is equipped with state-of-the-art research facilities that are open to all students on the island.

Juugo Yama was exiled from his home and forcibly sent to the island of Nanaejima to attend the local high school. With a limited budget, he chose the cheapest apartment he could find, but there was a catch. The apartment is haunted by Nanana, a girl that was murdered in the room. Nowhere else to go, he will now have to live with his ghostly roommate and venture into the world of treasure hunting with his classmates in order to release Nanana. [DrCoke]