The boy was a passenger in an SUV stopped behind the bus, Driscoll said. He declined to identify the child.
Two other people suffered serious injury in the crash, which happened about five miles south of Farmington Hill and a mile north of Sunnyside Elementary School.
The bus was stopped at Juniper Road and U.S. Highway 550 dropping off students shortly after 3 p.m. when the crash occurred. Two SUVs were stopped behind the bus when the driver of a Ford F-550 rear-ended one of the SUVs. The truck pushed the two SUVs into the bus, Driscoll said.
The driver of the Ford F-550 was identified as Matt Kibel, 31, of La Plata County.
Driscoll said charges are pending against Kibel; he is not suspected of speeding or using alcohol.
The 10-year-old boy killed in the crash was a passenger in a 2000 Mercedes-Benz SUV stopped directly behind the bus.
The driver of the Mercedes-Benz, Angelica Mannin, 32, of Durango, and a 4-year-old boy suffered serious injuries. Mannin was airlifted to Farmington and the boy was airlifted to Mercy Regional Medical Center, Driscoll said. He could not say how Mannin is related to the children in the car.
Michael Latham, Rachel Latham and four children, all of La Plata County, were in a 2004 Suburban stopped behind the Mercedes. They did not suffer serious injuries.
Twenty-two people on-board the bus, including 20 students from Sunnyside Elementary, a parent and a bus driver, were also unharmed, he said.
The students were taken to Florida Mesa Elementary School to be reunited with their parents.
The highway was closed about 3½ hours, Driscoll said.
Traffic was diverted to County Road 302, according to the Colorado Department of Transportation.
At first, the school district mistakenly notified parents of students on a different bus route, creating a slight delay in notifying the parents of students involved in the crash.
The mix up occurred because students who normally ride route SS3 were on route SS2, said Julie Popp, spokeswoman for the school district.
The district learned of the mix up from a parent of a student who was on the bus involved in the crash who had not received the district's alerts about the crash and reunification site. Upon learning of the situation, the district sent alerts to all parents of Sunnyside Elementary, Popp said.
Popp said it took the district about 20 minutes to identify the error and resend the alert. She could not say why students normally riding the SS3 route were on the SS2 route. There may have been a problem with the original bus, she said.
Jen Wilfong said she learned of the crash from her daughter, Sydney, 10, who was on the bus with other students from Sunnyside Elementary. Wilfong expressed concern about the slow alerts from the district.
"Nobody knew what was going on," she said.
Wilfong called various school district offices until reaching the district's bus barn which informed her of the reunification site.
"We didn't know for a good hour where our kids were," she said.