First Love (2002)

Kasumi is shocked to discover that her older sister's fiance is the man she first fell in love with five years ago in high school. At the time, he was her high school teacher and ultimately quit his job racked with guilt over their relationship. Kasumi was deeply hurt and struggled for a long time to overcome her sadness. Now the two of them must come to terms with their secret past before the ones they care about in the present are hurt.