Investigators have not pinpointed the time of the accident but say the group left Camarillo on Saturday night. The wreckage was discovered Tuesday morning by a passing trucker.
The dead students were identified as Trevor Beasley, 16; Matthew Daro, 16; James Saltee, 17; and Pamela Joy "Candy" Legaspi, 16. All of them lived in Camarillo, where the three boys had been close friends since kindergarten.
Under glowering skies Wednesday, the school went about the business of grieving.
"The day is very somber, just like the weather," Assistant Principal Laura L. Rynott said. "It's difficult to put your mind around something like this."
Seven district counselors and a school psychologist were stationed in a campus building. A youth pastor and a private therapist also volunteered their time. Both students and teachers, some in tears, waited to see them.
"Whether you lose one or 10, you can't quantify the loss," Rynott said. "So many people knew them or their brothers or their sisters or their friends. I have a secretary who grew up with one of their moms. It's very difficult."
In Mission Oaks Park, about a dozen of the students' friends put up a makeshift memorial, tying red, heart-shaped balloons to a picnic table and gently propping flowers and photo-covered poster boards against a gazebo.
The three boys, enrolled in the high school's independent-study program, lived in the nearby neighborhood. Over the years, they grew to love the same quintessentially California pastimes: surfing, snowboarding and skateboarding.
"I've known them since I was a year old," said Peter Mamakos, 16. "We were the best of friends and they were gone in, like, seconds."
He said he phoned his friends about 8 p.m. Saturday when they were at the Camarillo In-N-Out hamburger restaurant. They seemed hesitant about heading up to the big street party 150 miles away in San Luis Obispo, Mamakos said.
Even so, they set out, following a car driven by another teen. The driver of that car, who asked that her name not be used, said she sped up the coast, stopping in Pismo Beach about 9:30 p.m. to telephone her friends, who were lagging miles behind. She warned them to take care; police cruisers were all around, trolling the highway for speeders.
Her friends never made it to the party.
What happened isn't known. With Legaspi at the wheel, the Mercedes seemed to have been traveling at about the 65 mph speed limit, said California Highway Patrol Officer Ernesto Sanchez, an agency spokesman. Investigators found no evidence of drugs or alcohol, he said.
Changing lanes on a curved stretch of the northbound 101 just past the exit for Highway 154, the car left the road, tumbled 15 to 20 feet down a slope, flipped and hit a tree.
"They were instantaneously killed," Sanchez said, adding that the force of the accident dislodged the car's occupants from their seat belts. One of the boys was thrown clear of the car.
The investigation may take several weeks, Sanchez said.
On Wednesday, grieving families began planning their children's funerals.
Friends and relatives of Candy Legaspi trooped into her mother Priscilla's house in a gated hillside subdivision. Priscilla Legaspi, a dentist, moved to the U.S. from the Philippines when Candy was 6. The family has lived in Camarillo for seven years, but Candy's father still resides in the Philippines. Frantically, the family tried to locate him Wednesday to notify him of his daughter's death.
Candy was a dedicated athlete, going out for her school's basketball, track and swim teams.
"She was very full of life and had a lot of friends," said her cousin, Anna Diaz. "She really got along with everybody."
Friends said the same of James Saltee. His family members declined interviews.
At Matthew Daro's house, his father, Eliot Daro, called him "an athletic, handsome, beautiful, sensitive young kid."
"Out of all my sons, he was the one I thought would be the golden boy," said Daro, a TV writer and producer. "I thought things would come easy for him, that life would treat him well."
After his mother died when he was 10, Matthew struggled, Daro said. But he poured himself into his surfing, basketball and his daily after-school skateboarding sessions with his two best friends, Trevor and James.
When his son failed to return home Sunday, Daro thought he knew why: Mardi Gras went on until Tuesday and Matthew wanted to stick around for it, regardless of the consequence.
"OK, 'he's not calling,' I thought," Daro said. " 'He knows he's in trouble. They're just going to play this out.' "
At the Beasley home, some of Trevor's friends filed into his bedroom, leaving handwritten notes. His father, John Beasley, wept as he talked about his son's Mardi Gras plans.
"I specifically told him not to go," Beasley said.
At 9:15 p.m. Saturday, Beasley called his son's cell phone and told him to be home no later than midnight. About 20 minutes later, Beasley called again. There was no answer.
Worried, Beasley hopped into his car and drove the 101 to San Luis Obispo, keeping an eye out for accident scenes. He did the same on the return trip. After driving for hours, he had seen nothing. Early Monday, he filed a missing-person report with San Luis Obispo police.
Trevor's mother also had forebodings about her son's Mardi Gras trip.
"The kids were pulled by the parties," she said. "You try and control them, but you can't. They feel like they're invincible."
Times staff writer Greg Griggs and photographer Spencer Weiner contributed to this story.