Have you noticed that musicians who take private lessons are usually way better than musicians who only learn music in a group, such as band or orchestra class?

I claim that the same is true for math.

I claim that if you were to go to lots of schools and ask the students who among them is really good at math, and then talk to the math whizzes so identified, I would wager that almost all of them have or have had private lessons of some sort, either from a parent, a tutor, or a teacher spending time after school with them.

Why do I claim such a thing?

Because given the generally poor quality of math education in schools, a student without private lessons will generally:

Private lessons with a qualified teacher give a student a chance to get a real math education, which most schools cannot offer.

That is why the claim is plausible. Is it true? Contact me if you have the expertise in empirical research needed to find out. Contact me if you are interested in funding somebody to find out.