My Little Bride (2004) on Netflix

Plot summary

Bo-eun is 15, dividing her attention between schoolwork, her friends and a crush on Jung-woo, the ace of the school baseball team. Her grandfather, bound by an old promise to Sang-min's grandfather from the Korean War, orders her to marry Sang-min, a man in his twenties. Both young people object, but the grandfather's authority carries the wedding through. Bo-eun keeps her marriage a secret at school and starts dating Jung-woo, sure she can handle both. Her friend Hye-won is envious, since she likes Jung-woo too. Sang-min then appears at Bo-eun's school as an art teacher, and the two must hide what they are from students and staff.