It was some time ago that I started reading yet another " my world suddenly turned into fantasy-like survival game" but I cannot remember the name - all I can remember was it was about a high-school boy who had hidden his strength even before everything began. At the begging of the novel the classroom suddenly locks and the only way to get out it two fight in a boxing ring that appears in the middle of the room. On of the students goes up first and starts making other students go up and kills them incredibly fast and mercilessly and it is soon discovered that when they step into the ring they get powers and points and the more they kill the more points they get. And the winners get to leave the classroom but after that there is a "second stage" and that is all I can remember. If anyone has read the novel and has any idea what it was called, please leave a comment