The accident happened just after 8:40 a.m. as the bus carrying 50 students overheated on Highway 18 on its way to Mary Putnam Henck Intermediate School in Lake Arrowhead, authorities said.
Three students suffered minor burns, while six more complained of difficulty breathing. They were taken to hospitals.
Firefighters were later called to the school after several students who rode the bus complained of itchy skin and breathing issues.
"It got into the heater and it started to steam," said 11-year-old Joey Frisbie. "Everyone was breathing it in the bus."
"Some of the fluid that leaked out, it's half water, half antifreeze, so some of that got onto the student's backpacks," said Donna Kellogg, interim superintendent for the Rim of the World Unified School District. "The students who did come to school this morning, the backpacks rubbed on their legs and they experienced some skin irritation."
All of the injuries appeared to be non-life-threatening.
A clamp to the bus heater core came loose, according to district officials. School officials said the anti-freeze is a non-toxic blend the district switched to in 2007.
Parent Shannell Frisbie took her two sons to the hospital to be checked out. She said her 13 year old complained of burning lungs, while her son, Joey, had a visible rash on his arm and face.
"The protocol should have been to immediately get the children off the bus," the mother said. "That was what was most irritating part to me, is that I didn't know my children had been affected in any kind of way."
The driver may have been reluctant to have the students exit the bus because Highway 18 is a well-traveled and narrow road.
District officials said they tried to notify as many parents as they could and get students to safety.
The Associated Press contributed to this report.