Weak Hero Class 2

In shark the ML gets revenge on his bully and is sent to prison where he gets mentored to learn how to fight. Both shark and weak hero 2 have the aspects of school fighting with both sharing a crazy villain character

Two friends, Kyung Min and Detective Jong Suk, reconnect after receiving a message from a friend they haven't heard from in 20 years. The message triggers painful memories of their school bullying, and soon after, a series of mysterious murders begin. As Jong Suk investigates, he uncovers connections to the past, while the drama explores the origins of school violence and its enduring impact on society.