Steven Clavijo, a high school senior from Santa Clarita who planned to enroll the school, was trying to catch a nap when he felt the vehicle begin to shake from left to right and then he heard a loud boom.
"We knew we were in major trouble," he said.
After he escaped, two more explosions soon followed. Clavijo and other survivors looked on knowing others were still trapped in the inferno.
Both drivers were killed, along with three adult chaperones and five teenage students, according to the California Highway Patrol.
Towering flames devoured both vehicles just after the crash, and clouds of smoke billowed into the sky until firefighters doused the fire, leaving behind scorched black hulks of metal. Bodies were draped in blankets inside the burned-out bus.
The National Transportation Safety Board sent a team to investigate.
"Every piece of paper associated with this will be looked at," said Eric M. Weiss, an NTSB spokesman.
The crash happened a little after 5:30 p.m. on the interstate near Orland, a small city about 100 miles north of Sacramento.
A first responder who helped set up a triage at the scene said 36 or 37 people were hurt, with injuries that included burns, broken legs and noses and head lacerations.
"The victims were teenage kids. A lot of them were freaked out. They were shocked. They still couldn't grasp what happened," said Jason Wyman of the Orland Volunteer Fire Department.
Eleven people were taken to Enloe Medical Center in Chico. Two of those patients were listed in critical condition Friday morning, the hospital said.
At least seven people were hospitalized at other facilities. Two of them were in critical condition and five others in fair condition.