My Little Bride (2004) on Netflix

Boeun (Moon Geun Young) is an ordinary high school girl who worries about grades and has a crush on her school's baseball team ace, Jungwoo. One day, Boeun's grandfather orders her to marry Sangmin (Kim Rae Won) because of a pact he made with Sangmin's grandfather during the Korean War. Despite the grandchildren's opposition, they are forced to marry because of Boeun's grandfather's strong influence. Boeun's undercover married life begins: She pretends that she doesn't have a husband and starts dating Jungwoo. Boeun believes that she can manage both men and live a double life. Everything goes smoothly until Sangmin visits Boeun's school as a student teacher. There a teacher called Miss.Kim flirts with Sangmin and she later finds out that Sangmin and Boeun are married.She punishes Boeun by making her paint a huge wall of a stage.Sangmin then helps Boeun with the work of painting without letting her know.When the entire family are having their monthly get-together, Boeun's elder brother accidentally shows a video of Boeun with Jungwoo.Then after that she realises her mistake of cheating Sangmin and apologizes to him.She also learns from her friend that Sangmin had been helping her.She then breaks up with Jungwoo.During a cultural festival at Boeun's school where Sangmin is asked to make a speech,it is revealed to the entire school that Sangmin and Boeun are married.Sangmin then tells everyone that Boeun was forced to marry him and begs everyone not to discriminate her and treat her just like others.Now, Boeun comes to the stage and tells Sangmin that she is not a kid anymore and tells that she loves him.The movie ends with Sangmin and Boeun living happily together.