"It is painful to see those tiny bodies in that state," Lesufi said. He reported "scores" trapped but an Associated Press witness later said no one remained in the rubble. "When we arrived we found that there are still children underneath," said Kay Makhubela, spokesman for Gauteng police. He said about nine people were seriously injured.
The students at the school are between 13 and 18 years old, local broadcaster eNCA reported. It was not immediately clear what caused the collapse. Police said engineering reports would form part of the investigation.
Local media posted photos of a large slab and scattered bricks with the scene blocked off by emergency tape. The national Department of Education extended condolences to the families of the students killed. Young boys from a neighboring school came by to lay flowers.
"My heart goes out for the school because a lot of my friends were (there)," said one of the boys, Herco Lintvelt. "I hope they can build the school up again. And I hope everyone who is here must be fine."
A statement by AfriForum, a civil society group that represents the rights of Afrikaners, sent out a notice urging people to avoid the area so as not to interfere with the work of emergency responders.
Associated Press video journalist Nqobile Ntshangase in Vanderbijlpark contributed.
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