Ranulfo Marquez, head of regional rescue services, initially told W Radio that 63 people had died in the crash. "Three (men) survived, they are in critical condition." Later officials revised the death toll down to 57.
Mr Marquez called the accident among the worst along this stretch of roadway, considered one of the most notorious in Mexico for its treacherous curves and steep ravines.
The bus plunged 200 meters down into a ravine and rescue workers struggled to retrieve bodies from the remote location.
Emergency officials said three people were seriously injured but were unable to give statements.
"A lot of bodies, mostly of children, were flung all over," said David
Betanzos Espinoza, civil defense director in the nearby town of Ixtaczoquitlan.
The accident occurred near the town of Maltrata, some 230 kilometers east of the Mexican capital on a winding highway between Mexico City and the port of Veracruz on the Gulf of Mexico.
"The speed of the bus may have been for lack of brakes," said Arturo Corona of the federal police.
The bus apparently took a runaway ramp designed to stop vehicles with failed brakes, but missed a sand pit in the middle and ran alongside it instead.
The bus then jumped a guardrail at the top of the ramp and plummeted into the ravine.
Traffic accidents claim dozens of lives in Mexico during the busy Holy Week vacation, and as families travel home on the Monday.