The trip was a rite of passage, the students' last chance for all-out fun before preparing for grueling, all-important university entrance exams over the next two years. And Si-chan was excited.
While packing, he kept saying, "'What am I missing?'" his father, Joseph Park, recalled. "He was feeling so proud to be all grown up. This was supposed to be an adventure."
Before the 17-year-old set off, the elder Park offered some advice. He told his son to be sure he knew where the life jackets were, "just in case." He added: "If anything happens, just do what those in charge say. Listen to them, and you'll be fine."
That's exactly what the kids and other passengers did, and many paid with their lives when the five-story-high Sewol ferry turned upside down and sank in just a few hours on April 16. Most of the 304 people dead or missing were teenagers trapped in cabins where the crew had ordered them to stay.
The tragedy, one of the worst maritime disasters in this country's history, has prompted Koreans to question the very foundations of their society, their strong faith in authority, and the price of the rapid growth South Korea has seen over the last half century.
"For us, it's a question of trust," said Changsu Han, a psychiatrist at the Korea University College of Medicine, which treated more than 70 students who survived the ordeal. "We are realizing the ties that bind our society together - justice, ethics, our moral system of social security - are not as strong as we thought they were."
The victims "listened to authorities, and did as they were told," he said. "They put on their life vests, and waited to be rescued. ... And many of them are waiting there still - at the bottom of the sea."
On the morning of April 15, one father, Lim Hee-min gave his 17-year-old son Lim Hyun-jin 200,000 won (about $200). But the teen - a serious, bespectacled boy who sometimes stayed up until 2 a.m. studying - took only 50,000, saying he might lose the rest.
Park Jong-dae also gave money to his son, Park Su-hyun, a bright 17-year-old who loved to play music on a black Epiphone guitar. The boy said he couldn't spend it all. He left about $50 on the dresser in his room.
The kids headed to Danwon High School in Ansan, an hour's drive south of Seoul. They wore their uniforms - ties for both boys and girls - and brought their luggage.
After lunch, Su-hyun asked his mom to bring anti-sea-sickness tablets to the school.
"Have fun! Be safe!" she called out when she left.
"OK, Mom!" he said, smiling as he ran off.
The students gathered at Incheon port for their overnight voyage to Jeju island. Heavy fog delayed the trip by two and a half hours, and some students thought it might be canceled.
"Why don't you just take a cab and come back?" Lim Hee-min told his son by phone.
"Everybody is going, Dad, the entire class," Hyun-jin replied. "I have to go."
Their ship was a white, 6,852-ton ferry that could transport nearly 1,000 people along with hundreds of cars and dozens of shipping containers. A redesign two years ago by the owner, Chonghaejin Marine Co. Ltd., had added extra cabin space but also made the ferry more top heavy.
Paperwork filed by the captain, Lee Joon-seok, showed the Sewol was transporting 657 tons of cargo and 150 cars, according to the coast guard. But the company that loaded the vessel has said it was carrying more than 3,600 tons - three times as much as the limit stated in a regulator's safety report.
The ferry finally sailed at 9 p.m. with 476 people on board, including 325 students and 14 teachers. An hour later, the kids watched a dazzling fireworks display staged from the stern.
Ko Kyoung-jin, a truck driver who took the Sewol three times a week to deliver packages to a post office, was hoping to get to bed early. But from his third-floor cabin, he could hear loud Korean pop music blaring.
The students were partying on the floor above, dancing, hollering and doing the limbo.
Lim sent a text message to his son. "When are you going to go to sleep?"
"A little late," Hyun-jin replied.
"OK ... good night," his father wrote. "I miss you, my baby."
The next morning, the ship's cafeteria served a breakfast of sweet-and-sour pork and kimchi. Some students came with their hair, still wet from showering, wrapped in towels. Girls were putting on makeup, and boys blow-drying their hair.
Out on deck, the sky was overcast and gray. Strong winds were blowing across the water. Hyun-jin took photos of himself, wearing a gray hoodie, with friends. When he sent the last one to his father, at 8:52 a.m., Jeju was about three hours away.
On the bridge, third mate Park Han-gyeol ordered a helmsman to make a 5-degree turn. That's when something went gravely wrong.
Tracking data show the ship made a 45-degree turn instead. It is unclear why.
Ko, the driver, was on a back deck when he heard several shipping containers crash down on the Sewol's bow. Students on the deck above screamed. In the cafeteria's kitchen, pots and pans, utensils and chairs crashed to the floors.
Passengers who had slept late emerged, wide-eyed, to see what had happened.
"Don't worry, there's no chance it will sink," said a truck driver on deck with Ko.
The ship was already leaning so far to the left, it was difficult to walk. Park Su-hyun began filming with his cell phone from his bunk bed, as other kids cracked jokes.
"Am I really going to die?" one student asks in the video to the sound of laughter. A boy says, "it's going to be ... hilarious if we upload this on Facebook." Another adds, "It's like we're becoming the Titanic."
One by one, though, the students unpack life jackets still wrapped in plastic and put them on, saying they feel dizzy and weak-kneed.
At one point, somebody takes Su-hyun's phone and films him. He grins and holds up two of his fingers in a victory sign. When he flatly says, "I love you Mom. I love you Dad. I love you both," it is clear he does not realize those will be the last words he speaks to his parents.
The first distress call came from a student who called an emergency number at 8:52 a.m. The Sewol issued an urgent radio message three minutes later.
"Please notify the coast guard. Our ship is in danger. It's listing right now," one crew member said. "Please come quickly."
At 9:00, the crew told a marine traffic center, "It's hard for people to move." By 9:17, the vessel had tilted 50 degrees. But the messages that boomed through the ferry's loudspeakers repeatedly told people to stay where they were and not to move. Many were in their cabins or common rooms inside the ship, which was becoming increasingly difficult to escape.
Lim Hyun-jin sent his father a blurry image of students in life vests leaning on the floor with their feet propped against the wall - which because of the tilting was becoming the floor.
Jolted, Lim Hee-min called immediately. His son answered calmly.
"Dad, I think the ferry is sinking."
And then the line went dead.
At 9:32 a.m., Lim sent his son a message: "Hyun-jin are you OK?"
There was no answer.
A few minutes earlier, at 9:23 a.m., a large red tanker radioed. "We are right in front of you. We will stand by. ... We'll rescue them."
Capt. Lee Joon-seok hesitated to order passengers on deck, where they would have had a greater chance of escaping. He said he waited half an hour to order an evacuation because of strong currents and freezing water. Surviving passengers have said they never heard an order at all.
At 9:37, in the Sewol's last communication, the crew said the ship had tilted 60 degrees. Only one of the vessel's 44 life boats was deployed; the severe listing quickly made the rest impossible to use.
Three buzzing helicopters plucked people off the right-hand side of the ship, which by then was facing the sky. Inflatable Zodiac rescue rafts picked up people from the railings and others who had jumped into the water.
Some crew members tried to smash windows and save passengers, including a young contract worker who gave her own life jacket to a student and died. But the captain was among the first to escape. Video footage released by the coast guard shows him barefoot, in dark boxer shorts and a black jacket, sliding down a rope to board a coast guard vessel.
Ko, the truck driver, waited until the last moment, climbed down the railing and plunged into the 12-degree Celsius (53-degree Fahrenheit) sea. "I'm safe. I'm alive," he thought to himself as he was hauled into a Zodiac.
When he looked back, the ferry's entire left side was submerged. Water was pouring into the bowels of the ship, sealing doors shut - including those of the students still huddled in their cabins.
One hour later, the Sewol was upside down, its blue hull rising above the waves.
In Ansan, the school issued a text message to parents at 11:06 a.m., saying all students had survived and were safe. Lim Hee-min felt hopeful enough to upload an image of his son posing on deck to Kakao Story, a Korean version of Facebook.
"On his way to Jeju," Lim wrote. "Everyone has been rescued, but I'm still worried."
Survivors were being brought to a school gymnasium in Jindo, a town five hours away from Ansan.
When Joseph Park arrived, people were scattered around on the wooden floor, in tears. A white board just inside the entrance listed confirmed survivors. Park Si-chan's name was not there.
"I so desperately wanted my son to be on the list," Joseph said later. "I cannot tell you how much."
After nightfall, police escorted some parents on boats to view the rescue operation. People shone flashlights into the dark water.
"Please save my son!" Lim screamed in the darkness.
In those first few days, there was still hope the kids might be alive. The rescuers used hoses to pump air into the ferry. But the weather was so bad, the currents so strong, that it took several days for divers to even get inside the ship. When they did, they brought out only bodies.
On April 19, Park Su-hyun's father, Park Jong-dae, posted a letter to his son on Kakao Talk:
"It must be very dark and cold there."
"I can't imagine how cold and scared you were."
"I can't imagine how furious you must feel hearing absurd stories of adults who do not have the will to rescue (you)."
"I sincerely kneel down and beg your forgiveness, as a helpless father who made you be born in this stupid country."
Three days later, on the night of April 22, a body was brought to port. The vague description posted by the authorities matched what Su-hyun had been wearing - a black T-shirt, black training pants, black Puma brand boxer shorts with a red band.
His parents pulled back the white sheet covering the body. Park's wife, Lee Young-ock, broke down in tears.
"My son, my baby. I love you," she whispered, leaning over to hug her boy one last time. "I'm so thankful that you were my son."
More than a month after the Sewol went down, at least 284 bodies have been recovered, and 20 more are still missing. The search has been delayed by bad weather, strong currents and visibility so poor divers have had to feel their way through the darkened ship by hand.
The victims include 250 students and 11 teachers. One of the three surviving teachers - the vice principal leading the trip - hanged himself from a tree soon after.
Investigators are still struggling to determine what happened. Prosecutors on Thursday indicted 15 crew members for suspected negligence, including the captain, and police have detained the head of Chonghaejin, the ship owner. The government is promising new monitoring and regulations for domestic passenger ships.
The disaster has come as an intense shock to this country of 50 million, a former backwater that has transformed itself into Asia's fourth-largest economy in just a few decades. The tragedy has led many to question how far South Korea has really come.
"Down deep, most Koreans knew their claims of rapid development were exaggerated," Tom Coyner, a Seoul-based editor of the Korea Economic Reader. "But few expected an incident of the scale of this tragedy to be rubbed in their faces as much as it has."
At Danwon High, black hearse limos drive down the hill out of the school, headed to funerals. A large sign draped beside the entrance, strewn with yellow ribbons, says, "It's not your fault."
Over coffee at a cafe nearby, Park Jong-dae removes his glasses. He rubs his temples, and speaks of those critical last moments in which his son and others might have escaped, but were told to stay put.
"The crew ordered them to stay inside the ferry. They told them they were safer there," Park said. "That's what I cannot understand. Our children didn't die in an accident, OK? They were murdered."
His wife, Lee Young-ock, said it is impossible to move on. "Every day I feel like my son will open the door and come home like he always does, smiling, telling stories from school."
The $50 that the boy left behind to spend after the trip is still sitting atop the dresser in his room.
Earlier this month, divers recovered the bodies of Lim Hyun-jin and Park Si-chan. Si-chan's father said he regrets the advice he gave his son to blindly follow authorities.
"It's important, in Korean culture, to listen to adults. My son listened well. He was obedient. And I praised him for that," Joseph Park said. "But if I could talk to him again, I'd tell him: Listen to adults, but it is equally important to think for yourself, to make your own decisions in life."