The minibus burst into flames after the collision.
Paramedics treated the injured at the scene after the van carrying students collided with a transport truck northeast of the capital, about 70km from Pretoria. The cause of the crash is unknown. Pieterse said paramedics on the scene transported seven students to the nearest hospital. A number of children had already been pulled out by "members of the community", the statement said.
ER24 said responders found about 13 children trapped inside the vehicle after its flames were put out.
South Africa's ER24 Emergency Medical Care agency confirmed the death toll stood at 20 children. However, the education department later clarified two of the dead were adults. The department said "this is a huge loss & we're deeply pained by the tragic news".
South Africa's ubiquitous minibus taxis are frequently involved in accidents on the country's roads.
Earlier the country's transport minister Joe Maswanganyi had called for greater caution on the roads as he announced that the Easter weekend death toll had increased by 51 percent to 235 compared to same period a year ago.