At least two school children were also killed in the incident, which happened on the R61 between Cradock and Graaf-Reinet in the Eastern Cape, Arrive Alive said.
"The principal was actually driving the minibus when the crash occurred," said Arrive Alive spokesperson Tshepo Machaea.
Fourteen people were seriously injured in the crash.
They have all been transported to hospitals in the area. The minibus was carrying schoolchildren to an event in Graaf-Reinet when it collided with a bus carrying passengers from Cape Town to Mthatha.
"The accident was a head-on collision," said Machaea.
It is still unclear what led to the crash.
Transport department spokesperson Ncebo Kumbaca said the deputy minister Sindisiwe Chikunga and Eastern Cape transport MEC Thandiswa Marawu were expected to visit the injured soon.
DA MP Donald Smiles, in a statement, extended his condolences to the families of those killed.
"Our hearts and prayers are with them," he said.
Smiles said the accident emphasised the need to pay serious attention to road safety.
"This heart-breaking incident could have been avoided. We cannot allow the unnecessary loss of lives on our roads to continue," he said.
Fourteen other people were seriously injured. They were being treated at various hospitals in the province. – Sapa