Six teenagers who survived South Korea's worst maritime disaster in 44 years told on Monday how classmates helped them float free as water flooded their cabins despite crew instructions to stay put even as their ferry sank, killing more than 300 people.

The teenagers, whose names were withheld to protect their privacy, were giving testimony at the trial of 15 crew members, who face charges ranging from homicide to negligence for abandoning the sinking ship.

"We were waiting and, when the water started coming in, the class rep told everyone to put on the life vests ... the door was above our heads, so she said we'll float and go through the door and that's how we came out," one of the teenagers said.

"Other kids who got out before us pulled us out."

The ferry Sewol sank on April 16, killing 304 people, as many as 250 of them school children on a field trip. Twelve of their teachers were also killed.

The ferry was on a routine trip from the port of Incheon south to Jeju island, carrying students and teachers from the Danwon High School on the outskirts of Seoul as well as other passengers and cargo.

Another of the teenagers told how crew members had told passengers, "specifically the students of Danwon High School," to stay in their cabins.

"Water started to fill in and friends helped us move out," the student said.

Others described how coastguard officers waited outside the stricken ferry for passengers to swim out rather than go into the ship to try and rescue them.

"They were outside. They pulled us (onto boats) but they didn't come inside to help," one said. "We said to ourselves, 'why aren't they coming in?'"

Another student said it appeared there were more fishermen involved in the rescue than coastguard. Like others, she said the crew should be punished severely for their actions.

"More than that, I want to know the fundamental reason why my friends had to end up like that," she said.

The six teenage survivors described how there were repeated orders not to move from their cabins. Orders to put on their life vests came much later and without any information about what was happening to the ship as it began to list sharply.

They were the first of 75 children who survived due to give evidence in the trial at the Gwangju court, which has been moved to Ansan south of Seoul to accommodate the students.

Five of them gave their evidence facing away from the court. One testified from another room via closed-circuit television.