My high school in a town-that-refused-to-admit-it-was-a-small-city had no course called "basic math". Compound interest was taught in exactly two classes: calculus and economics.

Economics was a de facto personal finance class, which was nice. It refused to admit that because "economics" looks better on a transcript than "personal finance". This meant it was a rarely taken elective for folks who wanted to look good to colleges.

Calculus was also an elective for folks who wanted to go to college, which was notoriously more difficult but more popular in spite of this. People not going to college weren't taking either one.

I believe grandparent that compound interest wasn't taught anywhere in a small public school. A lot of those schools don't even offer calculus.