A bus carrying teachers on a field trip in northern Thailand has slid off a ridge, killing 13 passengers and injuring at least 16.

Police Lt. Col. Kiattiyod Suwannakatsi says the driver was traveling fast along an unfamiliar route in a rural district of Nan province on Monday when the bus hit an electricity pole. The bus then fell into a 650-foot (200-meter) deep valley.

The teachers were traveling from nearby Chiang Rai province. Rescue workers spent more than six hours helping the victims out of the bus wreckage.