I do not know the numbers, but it is often noticeable at most boarding schools. I have a young child with kids in her grade sometimes 2 + years older. I think there are enough re-class children that it does not matter for the re-class students to fit in. It is actually harder on those that stay in their correct grade as many kids are older, more developed, and have often already experienced some of the curriculum giving them further edge, but this is not just with re-class children. It also happens with international students based on their schooling schedules. It can make clearer divides in maturity, interests, and successes.