It was "an absolute miracle" that no one was killed, Sgt. Paul Schroeder of the California Highway Patrol said after Tuesday night's accident along a remote stretch of Interstate 15.
The bus, operated by KT Bus Services of Las Vegas, was one of two from the David Cox Elementary School in Henderson, Nevada, that was returning from a trip to San Diego, Schroeder said.
The bus collided with the back end of the rig, hurling both vehicles down the embankment. The bus landed upright against the truck's undercarriage, scattering sleeping bags and duffel bags across the wreckage.
Rescue workers took almost an hour to reach the crash scene about 65 miles south of Las Vegas, said Dennis West of the San Bernardino County Fire Department. "This is out there in the middle of nowhere," he said.
The bus driver was reported in critical condition. Six people had moderate injuries and 33 children suffered minor injuries, West said.
Saint Rose Dominican Hospital treated 27 children and six adults. "Most just have bumps and bruises," said Rod Davis, hospital president. "They've been assessed, checked out and released."
A total of 77 students and 12 chaperones were on the two buses.
The highway was closed for about two hours in both directions after the crash, and the northbound lane was shut throughout the night.
Schroeder said investigators hadn't determined the cause of the accident.
"We don't know why, if it was fatigue or what. It's way too early to tell," he said.
Written by Angie Wagner