Jishou F-Rank no Oniisama ga Game de Hyouka sareru Gakuen no Chouten ni Kunrin suru Sou desu yo?

Shishio Academy is a prestigious school in Japan, in which only the elite of academics, sports, or pedigree can study. But, in reality, it is a school where "the strong devour the weak," where everything is measured by the results of the games. This is a massacre in which only a definite winner can survive. Guren is a legend of the shadow world, can invincible conqueror, and at the same time, he wants to start a normal, carefree life. He fails the entrance exams to the academy on purpose. Therefore, following his results, he qualifies as the lowest grade F. However, after reuniting with his younger sister Karen, the situation changes dramatically. When his sister is in danger, he shows his true abilities! (Source: Kadokawa, translated)