Khuselwa Rantjie said it was unclear how many children were on the bus, but three had so far been found alive. Rescue efforts had been suspended as night had fallen and would resume on Wednesday, she added.
In a separate incident, the bodies of seven people carried away by flood water have been found in the province's OR Tambo district.
South Africa has been hit by heavy snow, rains and gale force winds that have claimed the lives of a further five people in a road accident, and have left nearly 500,000 homes without electricity.
