Excluding private schools whose makeup differ mostly due to their location there are still a lot of schools that are predominantly white - most of them are Afrikaans medium schools located in predominantly white areas. These schools are on average I suspect better functioning and better funded than schools in other areas who are at the sole mercy of the government.
The reason they are better funded is because the parents can afford to pay higher school fees so the school can employ extra teachers and has money to spend on resources that the state does not provide.
These schools are the easy targets she is aiming for - show your average ANC voter the nice and shiny "white" school and if they compare it to the Teachers union infested state school where they live it makes it easy for her to blame the white school for excluding their kids.
It would be a lot harder for her to explain to her voters why after almost 30 years in control she has not fixed the state school, or why the thieving and corrupt ANC has not grown the economy enough for them to afford to pay the higher school fees needed to send their kids to a decent school.
Putting the severe lack of math skills aside, what she is actually saying is that she wants white schools to be forced to accept 50% black pupils. Any sane person can predict how this ends, the white parents who can afford it will move their kids to private schools, not because they are racists but because the strain that the people in the school who are unable to pay puts on the school will mean the school fees for the paying people will have to be raised to the same level as private schools or the schools will just fail in the same way the state schools do.
So not fixing anything, just breaking the few functioning schools that are left. The ANC has to be very careful; at the rate they are obliterating the last few vestiges of a functioning economy and society they will soon run out of examples of white privilege to point to.