At least 21 people were killed when a bus carrying students to their annual college entrance exam plunged into a lake in southwest China on Tuesday, authorities and state media said.

The bus crashed into a guardrail and veered into Hongshan Lake in Guizhou province, leaving 15 others injured, the local emergency management department said in a statement.

The incident took place around noon in Anshun city and eyewitnesses said the bus’ passengers included students who were on their way to sit their gruelling “gaokao” examinations, state broadcaster CCTV reported.

Security camera footage shared on social media by CCTV showed the vehicle veering across at least five lanes — cutting into oncoming traffic — to plough through barriers on the other side of the road.

All those injured in the crash are being treated in hospital. Local authorities pledged an investigation into the cause of the accident.

Guizhou’s emergency management department said more than 200 people helped with rescue efforts, including 55 professional divers. (AFP)