Flat-earthers believe the earth is round.  Round is a planar (flat) shape.  What they don't believe is that the earth is spherical.

I don't blame the educational system for the flat-earthers.  You can lead a horse to water, but you can't make it drink.  There is a subset of humans who will forever refuse to accept any evidence that contradicts their preconceived religious faith - faith in words written millennia ago by scientifically illiterate people who mistook the earth as flat.