"They kept saying the same thing over and over," one said, describing how she and classmates obeyed the order until the ferry had listed so far that the door to their cabin was above their heads.
Another described watching a wave sweep her classmates back inside the sinking boat.
The actual trial is taking place in the southern city of Gwangju, but the judges and lawyers decamped to a court in Ansan city, south of Seoul, for a special two-day session with the 17 students who agreed to testify.
Police cordons blocked public access to the district court as the students - all from Ansan's Dawon High School - arrived in a red mini-bus and were escorted into the building by a tight phalanx of police officers.
Although they were offered the option of testifying by video from a nearby room, five of the six female students involved in Monday's morning session chose to give their testimony in the courtroom.
The student who took the video option described how passengers suddenly slid to one side as the ferry listed heavily.
"The internal tannoy announcement said we should put our life vest on and stay put," she was quoted as saying by a pool reporter in the court, adding that the message was given repeatedly.
Of the 476 people on board the 6,825-tonne Sewol passenger ferry when it capsized on April 16 off the southern coast, 325 were Dawon High School pupils on an organised outing.
Only 75 students survived.
The female student said they had obeyed the order not to move until water started coming through the window of their cabin which, by now, was under their feet.
"The door was above our heads. We had our lifejackets on and the president of our class suggested we wait until we could float upwards and then escape," she said.
Eventually some classmates managed to clamber up fixed furniture. They pulled the others up and out as the waters inside rose.
Another witness, who testified in the courtroom, said at no time was she or those who escaped with her helped by any crew.
As the ferry keeled over to one side, she said a group of them managed to move along a now horizontal stairwell towards an escape hatch.
At the moment she jumped out, a sea swell swept over their escape route.
"There were many classmates in the corridor and most of them were swept back into the ship," she recalled.
The tragedy, and in particular the loss of so many young lives, rocked South Korea with an overwhelming sense of collective shock and grief.
In the days immediately after the disaster, TV stations broadcast wrenching mobile phone footage taken by one student victim of himself and his classmates laughing and joking about being in the Titanic movie as the ferry begins to list.
As the situation worsens, the students begin to panic, even as the ship tannoy can be heard advising them to stay put.
Sewol captain Lee Joon-Seok and three senior crew members are accused of "homicide through wilful negligence" - a charge that can carry the death penalty.
Eleven other crew are being tried on lesser violations of maritime law.
The bulk of the charges against the crew arise from the fact that Lee and the others chose to abandon ship while hundreds of people were still trapped inside the heavily listing vessel before it capsized.
The final death toll was just over 300.
The crew were also condemned for ordering the passengers to remain where they were when the ship began listing.
A handful of crew members who stayed and tried to guide passengers to safety were among those who died.
Lee and his crew were publicly vilified in the wake of the tragedy, and there have been some expressions of concern about how fair their trial can be with emotions still running so high.
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