Of the 43 teenagers on board, Los Angeles Unified School District officials said 19 were from 15 LAUSD campuses.
The students were on their way to tour Humboldt State University in Arcata when the bus and a FedEx tractor-trailer collided on the Interstate 5 Freeway north of Highway 32 near Orland.
The collision happened between Sacramento and Redding, west of Chico, at about 5:40 p.m. The FedEx truck was traveling southbound when it crossed into northbound traffic and struck a Nissan Altima, then the tour bus, California Highway Patrol officials said.
The driver of the Nissan Altima, Bonnie Duran, told KOVR-TV 13 the big rig was already on fire as it crossed the median and hit her and then the bus. However, this report has not been confirmed.
The bus that crashed was one of three heading to Humboldt, and had been traveling a bit behind the other buses, CHP officials said.
CHP officials said the cause of the crash remains unclear. A final report on the crash is not expected to be completed for another three to six months, CHP officials said.
National Transportation Safety Board investigators arrived on the scene around 10 a.m. Friday. CHP officials said the NTSB will reconstruct the crash scene with a 3D diagram as part of their investigaiton.
Investigators said the truck may have been trying to avoid the Altima when it collided with the bus.
The CHP said 34 people were taken to seven different area hospitals. Nine people were confirmed dead at the scene, and one person, Arthur Arzola, died at UC Davis after being airlifted there with severe burns.
Arzola, 26, of Rancho Cucamonga, was the first fatality to be identified in the crash. He was a recruiter for Humboldt State University.
Michael Myvett and his fiancee, Mattison Haywood, were also killed in the crash. Both were chaperones on the trip. The couple had just gotten engaged over Christmas in Paris, according to family.
Family members confirmed that Adrian Castro, an El Monte High School student, died in the crash. His aunt described him as a "fine young man" who played football and had a bright future.
Identical twin sisters from Norte Vista High School in Riverside made the trip in separate buses. Marisol Serrato, 17, arrived earlier in another bus. Marisa was on the stricken bus and confirmed dead by her brother.
Brother Miguel Serrato said medical examiners confirmed her death Friday through dental records.
One of the students on board the bus desrcribed his traumatic experience. Jonathan Gutierrez tweeted a photo of the crash scene, saying, "can't believe this just happened.. I was asleep and the next thing you know I was jumping out for my life."
Gutierrez spoke from his hospital bed to "Good Morning America."
"All I could think about was, what just happened, I need to get out of here. And it was hard to breathe in there, that's when I started panicking, the smoke was everywhere. You could not see where you were going, you saw the light from the window and you just hear everybody coughing. So traumatizing.
"The window to the floor down, that's a long jump. And people are just panicking and just throwing themselves out the window. I guess when I jumped out the window I fell on my leg, and that's how my leg got injured. I could hardly walk. But, I'm alive, and that's what counts."
The uncle of another student told Eyewitness News that his niece's teeth were knocked out and the family was en route to meet her.
Thursday night, Banning High School in Wilmington tweeted: "Our prayers go out our Banning Students who were in accident on the way to Humboldt State University and all the students in it as well."
The university had chartered the buses as part of its Preview Plus program, in which students from Southern California and the San Francisco Bay Area tour the Arcata campus, according to university officials. The other two buses did arrive, and the university was providing those students with counseling.
The university released a statement Thursday: "Humboldt State University is deeply saddened by a tragic accident that occurred earlier this evening involving a charter bus filled with prospective students. They were on their way to visit campus for the April 11 Spring Preview event."
Families and victims can reunite at Veteran's Memorial Hall in Orland, officials said.
LAUSD Superintendent John Deasy said the district is in contact with authorities and will not discuss any student names until families are notified if their student was involved in the crash.
Deasy said any parents needing assistance can call a 24-hour help line set up by LAUSD at (213) 241-3841. The help line will be in operation indefinitely. Concerned parents can also call Humboldt State University at (707) 826 6327.
The Associated Press contributed to this report.