The Gokusen (tv-series, 2004)

Kumiko "Yankumi" Yamaguchi enters Shirokin Gakuen Private School to become a math teacher. She's assigned as the homeroom teacher for class 2-4, populated by a gang of delinquents. At first, the class tries everything to rattle her cage, but to no avail. In a short period of time, Yankumi gains the respect of the class. But what the school doesn't know is that she's the heir to the Oedo Group, a powerful Yakuza clan. So while Yankumi struggles to excel in her career as a teacher, she must also keep her Yakuza background a secret.