Sang Doo! Let's Go To School (2003)

Cha Sang-doo (Bi) and Chae Eun-hwan (Gong Hyo Jin) grew up as the best of friends. One day, Eun-hwan's family goes bankrupt and debtors rushed to collect their belongings. When her father's old record player was being taken, Sang-doo goes after the guy, accidently killing him. After being released from prison, he finds that his family and Eun-hwan have left him. Years later, he becomes a gigolo (with his uncle) to pay off his sick daughter’s hospital bills. He eventually meets his long-lost first-love, Eun-hwan as a high school math teacher. In an effort to regain her love, he returns as a high school student in her class. As everything seems to be going smoothly, circumstances prevent them from staying togethe