Murder Lock: Satsujinki no Kyoushitsu

Koutarou Kudou was known as the "Streets' Strongest." Once, 13 thugs, thirsty for revenge, kidnapped his younger sister Hinami. Kotaro beat everyone, and on the same night, he was arrested on suspicion of murder, as each of them was later found dead. He is then forcefully enrolled to a certain school, in which all students must meet "a special criteria" to enter... (Source: Square Enix, translated)