A video of the state media showed that a bus carrying students for exams that suddenly drowned killed 21 and injured 16 in five other lanes with more than five traffic.
Video on CCTV's social media account showed the bus moving slowly along the roadway before turning 90 degrees, hitting a six-lane dual carriageway, and railing to the other side.
CCTV said some passengers on the government bus were "gaokao" or national college entrance exams.
State media say they are investigating the causes of the incident in Anshun in southwestern Guizhou province.
The student was one of 21 people killed and four of the 16 injured, according to the state paper The People's Daily. It was not immediately clear how many people boarded the bus or how many students they had.
State media showed pictures of a roadside bus sinking as recovery efforts continued.
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