Sachaejigan

Kim Sang-an is a cold-blooded loan shark. When his younger sister, a schoolteacher, takes her own life after relentless bullying from her students, he becomes the homeroom teacher of those very students to collect the price of her life. A merciless loan shark who will collect his debts even if it means descending into hell and a class of devil-like students... A vicious, twisted teacher-student relationship.

(Source: Naver, translated)