My Little Bride (2004)

Bo Eun is an ordinary high school girl who worries about SAT's and has a crush on her school's baseball team ace, named Jung Woo. One day, Bo Eun's grandfather orders her to marry Sang Min. When they were very young, Bo Eun's and Sang Min's grandfathers had promised that they would become in-laws. Despite their opposition, they are forced to marry because of the strong influence of Bo Eun's grandfather. Bo Eun's undercover married life begins: She pretends that she doesn't have a husband and starts dating Jung Woo. Bo Eun believes that she can manage both men and live a double life. Everything goes smoothly until Sang Min visits Bo Eun's school as a student teacher.