You should be able to get one by contacting your former school. It's not uncommon for Koreans to ask for such a letter or document to confirm when they worked at a certain company. The law says that former employers are obligated to provide proof of past employment.
Victoria Oh (Goyang District) - Goyang, Gimpo, Uijeongbu, Dongducheon-Yangju, Paju, Yeongcheon, Pocheon, Gapyeong, Bucheon ELEMENTARY AND MIDDLE SCHOOLS:031-900-2858, victoria@goe.go.kr