The bus plunged down a ravine between the towns of Pifo and Papallacta, about 60 kilometres east of the capital Quito, according to emergency services spokesman Ivan Collahuazo.
Forty-four bodies were recovered at the crash site and two people died later at a clinic.
Six children who survived the incident are in a critical condition, emergency services doctor Luis Arias said.
The bus, carrying 52 people, was on its way back to Quito after a trip to Papallacta, a region known for its thermal waters.