While Math is certainly cumulative, in general I don’t think this is true.
Both my kids were late bloomers and are both going to top schools.
My ability to pay for those top schools is probably more relevant in the result.
They both did OK as Fresh/Soph but hit their stride as Juniors in HS, getting As.
What they did all along was participate in very meaningful EC all along, from middle school.
But academically they went from good to great students, only Junior year.
One is in an ivy, the other in a top ranked tech school.