I grew up with a more complete history of the US thanks to all the Hawaiian history classes, which is why I recognized race (and racism) was literally created by plantation owners as a tool to keep working people from uniting in the cane fields by the time I was 14. The motherfuckers literally put these plans on paper for everyone to read, so it's not, like, a well kept secret or anything.
This is why white supremacists don't want actual history taught in class: it glitches the Matrix. It doesn't make "white" kids feel bad about their skin, it makes them realize that violent racism is as real and solid as a punch to the face. But race? Race is a fiction. And the next step from that point is the realization that there is no "white." There's only the plantation owner and there's the rest of us. And that's the implicit threat that's making him outlaw the truth.