Later in the day, fury erupted over the pace of the rescue operation.
Angry parents and students cursed and shoved reporters, photographers and TV cameramen, while about 10 female students wailed loudly and hugged each other. An unidentified middle-aged man shouted, "Let's smash their cameras the next time we see another flash."
The school, nestled in a quiet, clean residential area, was founded in 2005 and has more than 1,200 students and 85 teachers. The area is a half hour's drive from an industrial complex where many parents of students work at factories, according to residents. Ansan has a population of about 770,000, about 40,000 of whom are foreign workers from China, Thailand, Vietnam and other countries, according to city officials.
Many South Korean high schools organize trips for first- or second-year students, and Jeju is a popular destination. Students and parents in Ansan spoke of the excitement of the annual trip. The students have taken ferries to the southern island in recent years because they can spend more time with each other. They take pictures of the stars, of the ocean and islands, of each other. They gossip. They bond in a way not possible on shorter plane or train trips. Then, after the four-day trip is over, they fly back home.
Kim Eun-taek, an 18-year-old third-year student, knows more than 10 of the missing students, including one of his best friends.
"He lives next door. We used to play at my house together. He cooked food for me, fried sweet potatoes ... fried vegetables," said Kim, putting up his sweatshirt hood and lowering his head.
Ko Jae Hyoung, who sells fried chicken near the school, said the neighborhood is close-knit. Students grow up together, graduating from the same elementary and middle schools.
Ko closed his restaurant Wednesday to volunteer at the school. That night, he and about 30 others held a candlelight vigil to pray for the safe return of the students. Dozens of residents held vigils again on Thursday night.
Ko, whose daughter is a first-year student at the school, remembers some of the missing students visiting his restaurant to eat chicken and joke around with him.
"Now, the neighborhood is like a funeral home," he said.
Klug contributed from Seoul.