The school bus was speeding and carrying 52 passengers, even though it was licensed to hold just 26, the official Xinhua news agency, citing local authorities and a student who survived.
"The bus was going too fast," Xinhua quoted a third-grade student who was taken to hospital after the accident as saying.
The crash occurred about 50 kilometres (30 miles) from Harbin, the capital of China's north-east Heilongjiang province, according to Xinhua.
Eight students were confirmed dead and nine others were in critical condition in hospital, Xinhua said.
The agency said the bus driver did not have a license that allowed him to drive a bus.
Initial investigations also showed the owner of the bus did not have a license to run a service to carry students, it said.
China's roads are among the most dangerous in the world, with nearly 42,000 people killed in traffic accidents in the first six months of this year, according to government statistics.
Last year more than 100,000 people were killed on China's roads, an average
of 270 traffic fatalities every day.

Copyright AFP (Agence France-Presse), 2006