A state prosecutor confirmed "fatalities", after the crash, in which 43 others were injured. He says he thinks something hit a vehicle behind the bus.
The bus was left on the median of the highway with its front end torn off, and the dump truck was left with a mangled front end. A fifth grade class trip from the district was scheduled for Waterloo Village, a historic site about 5 miles away from the crash scene, according to the school's district's website.
The Democratic governor went to the school and says his heart is "broken by today's tragedy".
They later learned their classmates had been in a crash.
Mount Olive police said it was responding to the incident but would not give further details. At least one person died in the accident and several others are injured. He said he arrived before police, and drivers were running out of their cars to help crying and screaming children out of the overturned bus.
"We thought it was just a drill and people were joking around", said student Alejandro Garcia.
He said some students evacuated through the emergency exit on the vehicle's roof while others scrambled out of windows.
The crash left the twisted wreckage of the school bus lying on the roadside near Mount Olive Township, with its undercarriage and front cab almost completely torn off.
A dump truck sits near the scene after a collision with a school bus in Mount Olive N.J. Thursday
Multiple ambulances were on the scene and police said Route 80 was closed in both directions.
The school bus was carrying 38 students and seven adults, including the driver, from East Brook Middle School for a field trip to Waterloo Village.
Injured passengers were being treated at about a half-dozen hospitals in the region, Murphy said, including 21 who were taken to Morristown Medical Center and 10 who were transported to Hackettstown Medical Center.
A school bus traveling from East Brook Middle School with an unknown number of children collided with a red dump truck along Interstate 80 on Thursday morning. "Students will be dismissed from school".
But happiness turned to heartbreak and fear for these East Brook Middle School fifth graders.
Images from the chaotic scene in Mount Olive Township show the body of the bus shorn from its frame and pushed into a median guardrail, resting at an angle.
The dump truck was also damaged in the crash. He told HLN Thursday there were possible ejections in the crash. The company was fined $22,850 in 2016 for violating regulations on inspections, repairs and maintenance and post-crash drug and alcohol testing, according to the FMCSA.