"The victims are everywhere," said Cass County Coroner Gene Powlen.
The bus driver, identified by school officials as Debbie Duvall, was flown to a Fort Wayne hospital, where her condition was not available Friday night. The two truck drivers were not injured.
"It saddens the Twin Lakes School community to report the death of four of our students involved in a school bus accident today," said a statement issued by Superintendent Thomas Fletcher of the Twin Lakes School Corp. in nearby Monticello.
The bus was carrying special-needs students, the Logansport Pharos-Tribune, but Fletcher would not confirm that. The students attended two elementary schools in Logansport, the superintendent said.
The crash occurred about 3 p.m. on U.S. 24 west of Logansport, about 70 miles north of Indianapolis, said 1st Sgt. Dave Bursten of the Indiana State Police.
Troopers from the Indiana State Police worked to reconstruct the accident, interviewing everyone involved in the wreck, reports CBS affiliate WISH-TV in Indianapolis. Other emergency responders worked with parents who arrived on the scene to identify the students.
A mo-ped driver slowed suddenly in front of a dump truck, which tried to stop but clipped the bus in the oncoming lane, said state police Sgt. Tony Slocum.
The small bus flipped onto its side and slid into the path of a second dump truck, which struck its roof. The bus then slid into a ditch and came to rest, its front end and roof crushed.
The accident is being investigated.
Slocum said he did not believe the children were wearing seat belts, although one child was in a buckled-down wheelchair. Children are not required to wear seat belts on school buses in Indiana.
"I'm not a crash reconstructionist, but when you get struck in the top portion of the bus by a dump truck, I don't believe seat belts would have made a difference," he said. "The impact was just too severe."
Pam Simons, who lives next door to Duvall, described her as a longtime bus driver and said news of the crash was overwhelming.
"It's a huge tragedy for this community," she said. "All I can say is she's a wonderful person, a tremendous neighbor, and we're all praying for her."
The collision comes just days after police said a tractor-trailer driver on his cell phone hit a school bus in Florida, killing a 13-year-old girl.