Initially 15 people were thought to be trapped underneath the rubble and authorities pulled 14 individuals out from the debris, according to CCTV.
The collapse at the No. 34 Middle School in Longsha District in Qiqihar, located in northeast China's Heilongjiang province, was reported late on Sunday, according to the provincial fire and rescue department, Xinhua said.
There were 19 people at the gymnasium when the accident occurred. Four people escaped and 15 were trapped, the municipal search and rescue headquarters said, according to state media said.
As of 0500 AEST, 14 people had been pulled out of the rubble, with three showing no vital signs. Six died after treatment failed, state media reported.
An overhead ariel view of the scene from social media pictures showed a completely collapsed roof with rescue workers in the gym next to large boulders of concrete.
Other pictures showed large cranes hoisted on the side of the school building as rescue efforts were still ongoing.
The region and several parts of China had heavy rain this weekend, causing flooding and damage in some areas.
A preliminary investigation found that construction workers illegally placed perlite, a mineral with high water content, on the roof of the gymnasium during construction of a teaching building adjacent to the gymnasium, Xinhua reported.
Under persistent rains, the perlite soaked up water and gained weight, resulting in the roof collapse, state media said.
An in-depth investigation is ongoing and individuals in charge of the construction company have been taken into police custody, Xinhua said.
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