The final-year students from Danwon High School in Ansan city just south of Seoul returned to their classrooms which have remained empty since the ferry carrying 325 students on a school trip to the southern resort island of Jeju capsized and sank on April 16.
Junior classes will start next Monday, a provincial education official said, adding that he was "unsure" when the surviving members of the 11th grade class -- which was taken on the Jeju trip -- would resume their studies.
More than 12,000 mourners visited the memorial when it opened on Wednesday.
Danwon High students made up the vast majority of the 476 people on board the Sewol ferry and an even larger portion of the victims.
Of the 174 people who were rescued only 75 were students.