State police in New York identified the victim as Chelssy Mercier, 14, and identified her as being from Quebec, without mentioning her hometown.
Authorities said Mercier died after being ejected and becoming trapped underneath the bus, which broke through a guard rail, fell on its side and slid down a grassy hill.
The Quebec teen was partially ejected from the bus and pinned underneath, said state police Maj. Richard Smith. Mercier had been travelling with her mother on the bus.
Smith said passersby tried to raise the vehicle using a jack and that a police officer used a defibrillator in an attempt to revive the victim.
The accident occurred on Interstate 87, also known as the Adirondack Northway.
Most of the passengers climbed through the shattered windshield and some got out through an emergency hatch in the roof, Smith said.
Two Canadian families travelling behind the bus pulled over immediately to help. A member of one of those families described the scene to the Press-Republican newspaper based out of Plattsburgh, N.Y.
Mathieu Donnelly told the paper many people were shaking and crying and that there was blood galore as he and his father helped the injured.
Smith applauded their efforts.
"Each family risked their own safety, travelled into the median and did everything they could to assist at the scene, including attempting to lift the bus off our deceased, by helping with CPR and they remained for a significant period of time afterward to help in our investigation," Smith said.
Smith said the injuries among the other passengers included broken bones and cuts. None appeared to be life-threatening. All 56 passengers — between six and 54 years old — were taken to hospital.
There was no apparent sign the 61-year-old driver skidded before the accident.
The cause remains under investigation.
Police said there was no reason to suspect the driver, who passed three different inspections in the past year.
"There is currently no reason to believe he had a problem with his driving credentials," Smith said.
The bus was travelling south from Quebec City when the accident occurred between exits 29 and 30 in North Hudson, about 200 kilometres south of Montreal.
Don Jaquish, emergency services director in Essex county, told the Adirondack Daily Enterprise that several people trapped inside the bus had to be removed by emergency personnel.
"I'm sure a lot of them crawled through the top hatches because the bus was on its side," he said.
Jaquish said the bus was filled with college-age students and their relatives.
Members of the RCMP and Quebec provincial police were dispatched to provide assistance and help translate for the largely French-speaking group.
Neither tour operator Jaimonvoyage.com nor bus company Autobus Fleur de Lys knew exactly how the accident occurred.
The tour operator's customer care director said the bus driver was experienced and "knows the route by heart."
Josiane Grimard said the company ferries customers between Quebec and New York City every weekend in the summer, so the driver was used to the trip.
"He's a man of experience, all of our drivers are experienced drivers," she said.
Georges Morissette, president of Autobus Fleur de Lys, based near Quebec City, said in a statement the driver has nearly 30 years experience and an impeccable driving record.
He has worked for Autobus Fleur de Lys since April 2012 and was getting back on the road after seven days off.
Morissette said he spoke briefly with the driver as he sat in the back of a police cruiser and that the man was distraught with what had occurred.
The driver had originally been scheduled to be on vacation, but did the company a favour by filling in on a route he knows well, Morissette added.
The vehicle involved is a Prevost luxury bus with a maximum capacity of 56 passengers. The bus will now be impounded and studied by state police.
"It was in perfect mechanical condition and had undergone a periodic inspection that confirmed it conformed with company standards and Quebec regulations," Morissette said.
Jaimonvoyage.com marketing director Nancy Trudel said the bus picked up travellers in Quebec City, Trois-Rivieres and Longueuil overnight before embarking for New York City.
Trudel said company owner Andre Poulin travelled to the accident scene to help passengers and provide whatever support he could.
"For us, our priority is the passengers," Trudel said. "But we don't know what happened or how the accident occurred."
Grimard said another bus was sent to New York to pick up those passengers who weren't as seriously hurt.
"We want them back in Quebec as soon as possible to be with their families," Grimard said.
— Written by Sidhartha Banerjee and Peter Rakobowchuk in Montreal
— With files from The Associated Press