Joost Zeelie is 56 years old, a former policeman, a self-taught maths teacher and the director of an unregistered private school – one of the dozens the education department has warned parents about.

“I love maths and I was a good maths student at school,” said Zeelie, who has been operating the Hanniel Primary School in Nigel, on Johannesburg’s East Rand, for the past three years.

He told Times Select he felt “compelled ” to help pupils. He taught mathematics to grades 4 to 7...