Sang Du, Let's Go to School (2003)

Cha Sang Du and Chae Eun Hwan were childhood friends until tragedy struck when Sang Du accidentally killed someone while defending Eun Hwan's family belongings. After serving prison time, he discovers Eun Hwan and his family have disappeared. Years later, Sang Du becomes a gigolo to support his sick daughter and unexpectedly reunites with Eun Hwan, now a math teacher. To win her back, he enrolls as a student in her class, but unforeseen circumstances keep them apart. (Source: MyDramaList)