I remember arguing with my history teacher in high school because I predicted exactly this, and he was sure all races would be holding hands in SA and singing kumbaya. I really wish I’d been wrong.
I talked with a white South African a couple of years ago at work about this subject. He was totally clueless. Most of his family still lives in Cape Town and he didn’t foresee any problems. I specifically asked him what would happen when Mandela dies and he thought I was nuts.
I’m guessing your teacher was a liberal who was deluded by his own fantasies about the reality of human nature. Like many conservatives, my views are quite Augustinian on human nature so I have no trouble believing South Africa would turn into the Zimbabwe style disaster it’s turning into now.