The Amazing World of Gumball (2011)

S5E21 The Cycle

Gumball, Darwin, and Anais uncover that Mr. Wilson has been bullying Richard throughout his life, starting from a humiliating prank in high school. The siblings attempt to help their dad cope with the situation, but when their attempts don’t succeed, they conclude that the only way to end the cycle is to retaliate against Mr. Wilson.