It was supposed to be their last bit of teenage fun.

The realities of life would descend on the 325 students from Danwon High School, near Seoul, in the next school year in the form of cram time: nonstop study to prepare for South Korea's notoriously tough university entrance exams.

A four-day field trip to Jeju, a volcanic island whose white beaches and waterfalls lure hundreds of school groups each spring, was the planned distraction. The students would travel on a five-level ferry named Sewol, meaning "time and tide." More than two-thirds of the passengers were from Danwon, adding a feeling of camaraderie to the adventure.