The children, ranging in age from 15 years to 21 months, were headed toward their home about two miles north of the crash site. The truck hit them from behind, pushing their car into the bus and causing the car to burst into flames, police said.
Everyone in the car was killed, including the 15-year-old girl who was driving illegally. All of the youngsters had been adopted by the same family and lived together, police said.
It was unclear why the children were unaccompanied.
Evidence from the scene showed that the truck, which was carrying bottled water, did not brake before hitting the car on the two-lane road, said Lt. Mike Burroughs of the Florida Highway Patrol.
The bus ended up 200 feet from where the car struck it, and the cab of the truck lay overturned near the scene, Burroughs said. The bus was at an approved bus stop, but it was not immediately clear whether children were getting on or off.
"It's a very chaotic scene," Burroughs said. "It's just a mangled, charred mess."
Nine students were on the bus, and three were thrown from the vehicle by the force of the crash. But the extent of their injuries was also unclear.
State police said three were seriously hurt and six others suffered minor injuries. Hospital officials said they received eight patients, including two in critical condition and three in serious condition, reports CBS' Cami McCormick.
The drivers of the bus and the truck were also taken to a hospital. The truck driver suffered minor injuries, and authorities planned to interview him. The bus driver was thrown from the vehicle, and her condition was not immediately known.
The car was driven by 15-year-old Nicki Mann, who was with siblings Elizabeth Mann, 15; Johnny Mann, 13; Heaven Mann, 3; Ashley Kenn, 13; Miranda Finn, who was either 8 or 9 years old; and Anthony Lamb, who was almost two years old. Lamb was in the process of being adopted, Burroughs said.
The bus was operated by the Union County school district, which has three schools from pre-kindergarten to 12th grade in the area about 60 miles southwest of Jacksonville.
Shonal Patel of CBS affiliate WTEV in Jacksonville reports that Lake Butler is a very small tight-knit community.
Florida Highway Patrol lieutenant Bill Leeper says the crash happened on a two-lane road, where visibility is good, reports CBS News corespondent Peter King.
The National Transportation Safety Board said it was sending a team of investigators.
Fatal accidents involving school buses are relatively rare. The National Highway Traffic Safety Administration reports that 71 passengers and 42 drivers been killed since 1994 in school vehicles – an average of about 10 people per year.