The driver, whom police identified as Larry Dale Duke, struck the students as they walked to Mae Hensley Junior High around 9 a.m. Wednesday. The victims were all 13- and 14-year-old girls.
Witnesses told investigators that Duke ran a stop sign in his raised Chevrolet pickup and dragged one girl under the vehicle before stopping.
"He ran over one of them like a speed bump," Nikalas Matthews, who saw the crash, told the Ceres Courier newspaper. "You could tell he wasn't paying attention at all."
Police said they're also investigating witness reports that Duke was either on a cell phone or wearing earphones when he exited his truck.
Duke denied that he had been wearing earphones in an interview with KXTL-TV on Wednesday night. He said he didn't know how the crash happened, but said he might have fallen asleep at the wheel.
"I'm so sorry this happened," Duke told KXTL-TV. "I didn't mean for it to happen."
Ceres police spokesman Jose Berber said Thursday that Duke has not been arrested.
Authorities have not released the name of the girl who died at the scene, but family and friends identified her as 13-year-old Danielle Tarancon-Leon.
The other victims' injuries ranged from minor to serious, and three of them have been released from the hospital, police said.
On Wednesday night, about 200 people gathered for a vigil at a nearby park for the victims.
"She was just a loving, spunky, fun, caring girl," Danielle's grandmother Trinidad Tarancon told the Modesto Bee. "It's overwhelming to see the crowd of people here that cared about her and are showing their love for her."