I failed (got an F grade) in high school math for two years, because of bad teachers and chaotic classrooms. Eventually I just stopped going to class, guaranteeing a failing grade regardless of whether I understood math. At the same time I got through two years of math classes (trig and calculus) at a nearby university. I did that on advice from my high school counselor, who then gave me math credit because she understood the difference between high school grades and learning.
Statistics and probability came later, you're right.
Basic understanding of statistics informs us that aggregate results do not predict individual outcomes. The aggregate poor math performance of American high school students does not predict what any individual may achieve (or fail to achieve). The overall poor math grades and test scores among American high school students could have any number of causes. Overwhelmed or disengaged teachers and a one size fits all curriculum probably have something to do with it.
My comment was intended to get the original poster to think about taking control over their education rather than expecting school to do that for them. If high school math classes aren't working then look into alternatives. The OP already indicated they care about learning math, but aren't necessarily motivated to achieve good grades to get into one of the "best universities."
Actually, that's precisely what it does.
https://en.m.wikipedia.org/wiki/Ecological_fallacy
I'm not even sure what statistics has to do with the OP's question or my responses. It only came up because you implied I don't understand "Bayesian likelihood" in your ad hominem.
My point is that a student can get a poor math grade or struggle in high school for reasons other than lack of ability or interest in math. One way to test that is to try learning math in a different environment (the personal example I gave). If a student is failing math one hypothesis is they are not able to understand math, or aren't applying themselves. Another hypothesis is that the teaching environment is less than ideal -- something Bayesian likelihood might support.
My life expectancy is 85. Does that mean I'm going to die at 85? Probably not. It means that, of the people of my age, gender, and race, the current statistically-based projections are that half of them will die by 85, and half won't. How does that predict the outcome of any individual?
Guy fails high school math, but yes, have him ditch school, drive his mom's car to a university and audit college calculus.