The crash happened on Highway 330 just before 9 p.m.
According to the San Bernardino County Fire Department, the bus, which was carrying a church group of 36 people, was heading home to Orange County after attending a camp near Angelus Oaks.
The group was with Our Lady of Guadalupe Catholic Church in Santa Ana, according to the Diocese of Orange, which released a statement on the crash Monday morning.
"The group attended a three-day parish youth retreat at Camp Nawakwa in the San Bernardino Mountains," read the statement. "About 125 people, including teens, retreat staff and volunteers, participated in the event, which started Friday and ended Sunday. After the retreat, attendees left in several vans and one bus - the bus that was later involved in the accident."
"We extend our deepest gratitude to the first-responder agencies for their prompt and professional response in safely evacuating passengers and ensuring they received proper medical attention. We will provide further updates as information becomes available. Until then, our entire Diocese of Orange community will keep all those affected in prayer, and we thank all for their continued support."
The county fire department said 26 people were treated, three of whom had major injuries.
Of the 26 patients, 20 were sent to area hospitals, all by ambulances. The rest declined treatment, the fire department said.
Witnesses say the bus caught fire before it crashed and flipped over.
"They were braking and then right when he hit this turn right here, you could tell it was about to flip, so he jerked it into the wall and tried to stop it, but it didn't obviously work out," one witness said.
Crews said everyone was able to get out, with some climbing through the top hatch of the bus.
The California Highway Patrol is the lead agency investigating the cause of the crash. Police say there's no suspicion of DUI.
The county fire department said in February, crews participated in a drill that featured a scenario involving an overturned school bus with adults and teens on board.
"Several of the rescuers on scene [Sunday] night participated in that drill, providing them with valuable experience in this kind of incident," said the fire department.