Gu Se-Ra

At Kangsan High School, one of the nation's most troubled schools, Gu Se-Ra is the top student. Her childhood crush, her older boarder, Lee Seo-Jun, comes to Kangsan High School for a teaching internship, and she experiences numerous hardships with the troubled students. To protect her crush, Gu Se-Ra resolves to transform this troubled school into a model one.
(Source: Naver, translated)