A bag that either fell off a vehicle or was thrown into the roadway caused the collision just before 10 a.m., Mahwah Police Chief James Batelli told Daily Voice.
The chief said 15 to 17 students were taken to The Valley Hospital with various bumps, bruises and minor injuries.
"Most complained of some type of pain," Batelli said.
A total of 94 students from School No. 11 were on a field trip to Woodbury Commons when the crash occurred, he said.
The assistant Passaic County superintendent of schools and Passaic Mayor Hector Lora were at the scene, the chief added.
Officers were headed to the area after a motorist reported the debris in the roadway when the buses crashed, he said.
"We haven't determined where the bag came from yet," Batelli told Daily Voice just before 11:30 a.m.
The bus company is out of East Orange.
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