Kurôzu zero (2007)

The Suzuran Senior High School for Boys, nicknamed "The School of Crows", is the poorest achieving, most violent school in the country. The students are called "crows" and they band together in factions, battling each other for influence and power. But they all share a common goal, one that has never been reached in the school's history: unification. No one faction has ever reigned supreme. Genji Takiya, a transfer student who attempts to take over Suzuran Boys High School and is mentored by old boy yakuza Katagiri as he faces off against rival Serizawa.