More than 300 students from Danwon high school, located south of Seoul, were on the Sewol ferry when it capsized.
Most of the students are dead, or missing inside the sunken hull.
The ferry sank last week as it sailed from Incheon to Jeju Island. More than 170 people have been confirmed dead, as search teams work to recover bodies.
There were 476 people on board, with many trapped inside as the ferry listed and sank within two hours of distress signals being sent. A total of 174 passengers were rescued.
Almost 250 students and teachers from Danwon have been confirmed dead or are presumed to have died, Reuters news agency says.
Most of the students who survived the disaster remain in hospital and it is not clear when they will return to school.
Final year students returned to Danwon school on Thursday, however. Dozens of counselors have been brought in to help with the trauma that many of the students are expected to face.
Over the past week, Danwon high school has become a memorial site – flowers and messages piling up outside the gates and funeral rites taking place in classrooms.
Messages posted in classrooms reportedly included: “If I see you again, I’ll tell you I love you, because I haven’t said it to you enough.”
Many in South Korea are worried about what this tragedy will mean for the children – the missing faces in Danwon’s classrooms and the knowledge of how fallible their protectors can be, our correspondent adds.
On Wednesday, an emotional memorial service took place near the school, with friends and family members laying flowers in front of photographs of some of those who died.
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