"not everybody is born with math skills" isn't the argument though. The argument is "not everybody has the same math skills". It's a multidimensional and continuous thing, but it's not exactly feasible to teach that way, so some approximation is involved.

It sounds like that was implemented very poorly in your case. You yourself said "I was immediately bored in our lower maths class". I don't see how the solution to this is to make everyone take the lower math class. Certainly there is some middle ground between the California plan and what you experienced.