Kangoku Jikken Vol.1

Eyama Aito, a high school student, is the victim of daily bullying. One day a strange letter arrives inviting him to a "captivity game." He may choose one person to hold captive for one month with a chance at winning a large amount of money. He chooses the ringleader of his bullies, Aya Kirishima, and the only rule of the game is to not kill her. Everything else is fair game. Eyama's revenge begins!