When Jo Gang-Ja attended high school, she was notorious for fighting. She gave birth to her daughter A-Ran in her late teens and became more responsible. Her daughter A-Ran is now a high school student, but A-Ran is bullied at school. Jo Gang-Ja decides to go back to high school to protect her daughter. Jo Gang-Ja becomes a high school student again.
Jo Kang Ja, once a feared troublemaker in high school, dropped out after becoming pregnant and now works to be a responsible mother to her daughter, Ah Ran. At 34, Kang Ja's relationship with her teenage daughter is strained, with Ah Ran embarrassed by her mother's unrefined ways.
When Ah Ran befriends the school outcast, Jin Yi Kyung, she becomes a target for bullies but refuses to tell Kang Ja. After discovering the bullying, Kang Ja takes matters into her own hands by enrolling undercover at Myungsung High as "Jo Bang Wool." As she confronts the bullies, Kang Ja uncovers deeper issues in the school system and, with the help of Ah Ran and the idealistic teacher Park No Ah, sets out to end school violence and corruption at Myungsung High.