In the U.S., I heard from a parent whose kids go to exclusive private schools that the schools will often hold new students back a year or even two, as they are

In the U.S., I heard from a parent whose kids go to exclusive private schools that the schools will often hold new students back a year or even two, as they are more likely to do better in class, do well on standardized tests, and exceed in certain sports - all of which burnish the schools' reputations and lead to better college placements, which often means Ivies and other elite schools. Another option for students who struggle at public or private high schools (and whose parents can afford it) is a "post-graduate" year at a dedicated private academy for 18 and 19 year olds. The main purpose of these academies is to bring their grades and SATs up to a good level so they are more likely to get in to a good college. I know two people who did this in the 90s, one of them got into NYU after the post-grad academy.