Spokesman for the disaster response command centre, Hao Bin, said the number of missing had risen to 24, all but two of them children.
“They have been missing for four days. They could have been washed away,” Hao said.
He insisted that the toll would not exceed 100 but failed to explain why.
Villagers said they feared several hundred children had died.
Newspapers on Monday showed soldiers trying to clean up the mud and debris.
State media also carried some of the first witness accounts of the tragedy, caused by a mountain torrent thought to be the worst in 200 years that smashed into the valley school.
The flood crashed down on the Shalan Township Central Elementary School in Ningan city, leaving many students aged six to 14 with no escape as the water level quickly rose to several metres.
Harbin is the regional capital
Of the 92 bodies taken to a local mortuary after Friday’s flood in Heilongjiang province, 88 were children, according to official figures.
Thirty more bodies were reported to have been sent to another funeral home, the China Daily said, but the report could not be verified.
Thousands of people perish every year from floods, landslides and mudflows in China, with millions left homeless.
Officials have said this year’s floods could be worse than usual, with at least 255 people killed last month, the start of the rainy season.