State media said on Sunday that rescuers were continuing their efforts, but at least 91 people were known to have died in Friday's disaster.
A wall of water and debris smashed into the school near Ning'an, in northeast Heilongjiang province. More than 350 children and 31 teachers were inside at the time.
Of the dead, 87 were primary school students and four were adult villagers, the Xinhua news agency said, adding that more than two dozen people were still in hospital.
The flooding, triggered by torrential rain, caused damage in seven villages near Ning'an city and knocked down 55 houses, the agency said.
Heilongjiang Governor Zhang Zuoji travelled to the site, about 450km from the provincial capital of Harbin, to help coordinate a rescue and relief force of 1400 people, it said.
Xinhua did not say whether the toll was expected to rise further, and a local official reached by telephone said news from the area was hard to obtain because of poor communications.
Chinese media reports
Chinese newspapers carried photographs showing the school half-buried by mud and water. Television footage portrayed brick buildings in the area with walls washed away and fields ravaged by floods.
The floodwaters had reached a depth of two metres as they swept through the school.
The school is about 450km from
Harbin - the regional capital
China suffers widespread flooding and drought every year, causing huge loss of life.
State media said that more flooding is likely with heavy rains predicted for northern and southern China in coming days, and several rivers swollen to dangerous levels.