Witnesses said the driver went around a closed barrier and straight into the path of the train in Cape Town.
He and another four children were injured. No-one on the train was hurt.
They were investigating a case of homicide against the driver. – the police spokesman said.
“The booms were down when the minibus crossed the level crossing. We are investigating a case of culpable homicide to establish whether someone can be held responsible for the deaths of the school children,” he added.
Stunned in shock parents were among the first to arrive at the scene. Families stood weeping near the smashed bus.
South African road fatality rates are among the highest in the world.