The bus carrying 61 students and teachers was returning to a high school in Depok outside Jakarta, the capital, late on Saturday from the hilly resort area of Bandung after a graduation celebration, said West Java police spokesman Jules Abraham Abast.
It sped out of control on a downhill road and crossed lanes, hitting several cars and motorbikes before it crashed into an electricity pole, he said.
Nine people died at the scene and two others died later in hospital, including a teacher and a local motorist, Abast said. Fifty-three other people were hospitalised with injuries, including some in critical condition, he said.
“We are still investigating the cause of the accident, but a preliminary investigation showed the bus’s brakes malfunctioned,” Abast said.
Local television footage showed the mangled bus in the darkness on its side, surrounded by rescuers, police and passers-by as ambulances evacuated the injured.
Road accidents are common in Indonesia due to poor safety standards and infrastructure.
Last year, a tourist bus with an apparently drowsy driver slammed into an advertising hoarding on a highway in East Java, killing at least 14 people and injuring 19 others.
In 2021, a tourist bus plunged into a ravine in the West Java hilly resort of Puncak after its brakes apparently malfunctioned, killing at least 27 people and injuring 39 others.