The Yiliang county government said on its website that the children were declared dead early Friday about 18 hours after the landslide smothered the school and three farmhouses. It says the missing person was in one of the farmhouses when the landslide struck after 8 a.m. Thursday.
More than 800 people living near a river blocked by the landslide were evacuated. The government says the landslide blocked a river, and its water pooled around the buried buildings, hampering rescue efforts.
Officials have yet to give a cause for the landslide but the area has been lashed by rain and is prone to earthquakes. A series of quakes last month left 81 people dead.
About 2,000 local people, medics, police and military personnel were trying to rescue the victims, the Yiliang county government said on its website.
Rescuers rushed from the Yiliang county seat to Zhenhe three hours away to dig out the students, said Peng Hong, an official with the county foreign affairs office.
Peng said the cause of the landslide was not yet known, but Yiliang was struck by an earthquake last month that killed 81 people and devastated several villages. In addition, rain has lashed the region of mountains and sheer valleys.
Though Thursday was a holiday across China, students were in school to make up for days missed after the quake, Peng said.