Shitsurakuen manga

Shitsurakuen summary:

Himoto Sora is a transfer student with a sense of justice and dreams of becoming a knight. Upon arriving at Utopia Gakuen, she learns of a virtual reality-like game played by the male students in which they use girls as weapons in battles for status and treat them as commodities. Seeing one of these girls being abused she steps in and through some sort of error unwittingly becomes the first female participant of the game.