Btw, as much as I know, basic set theory, math logic and proof techniques (e.g. "How to prove it" level roughly) are almost universally delayed till university years. These generic tools are much more important than calculus!
I don't really understand why. This results in many omissions in school-level curriculum, I.e giving formulas with no proofs. This results in kids seeing math as a kind of meaningless and boring symbol manipulation.
Was there ever a discussion around this problem in the US?
Besides, rate of change is high school material: physics and last year math lessons do cover it.
At least in eastern europe they do.