It's probably at least partly because at least some high schools literally teach students that income tax brackets mean all income gets taxed at that rate as part of their "civics" curriculum. Education, especially related to economics, is fully captured by far right cranks pushing propaganda through control over textbooks and program funding.

I even remember some dumbshit "educational" video from high school "civics and economics" that had some poor farmer having to not grow his herd because progressive taxation meant he'd be making less money if he made more. That class and the American History class I took the next year both stand out as just constantly getting basic material facts wrong. Like the English classes did a better job of covering history than the actual history classes did.

It's funny, I can only remember the names of two teachers I've ever had and I can barely remember anything of highschool, but I can remember these clear flashes of how completely dogshit most of the classes were, even two decades later.