I’ve no doubt that schools (and of course the students/parents) find it advantageous while they are in HS. If you can be ahead of other kids, more mature, more practiced - this is definitely an advantage.
(Apart from the issue of poor behavior when 19yo are forced to behave like younger teens in school, which is less likely at private schools that choose their students vs. in public schools where this happens when parents sometimes delay Kindergarten a year or two.)
But the question wasn’t how the students feel, or even their HS - it’s about the wider world, in particular college admissions staff. For most colleges, it probably doesn’t matter. But I emailed and asked some contacts of mine and it was unanimous:
- if the repeated year is omitted and later found out, that would be a huge red flag re. honesty
- a kid who repeats a year is seen as potentially less capable academically, even if more "mature" because they took five years to accomplish what other kid accomplished in four.
The reason that it doesn’t matter for most colleges is that all of these kids clear admission’s academic bars without much trouble. But when you have HYPSM looking with a fine-toothed comb, they will think that the kid who made it through “six years worth” of math in four years, is smarter than the kid who made it through “six years of math” in five years. And probably this is true. A student who doesn’t have the kind of 9th grade that s/he thinks could get him/her into HYPSM may actually not be HYPSM material.
As for the rest of the world - it is considered a “choice” to repeat a private school HS year that only the elite/rich can make, and that do-over is seen as an unfair advantage. You could argue with me about other unfair advantages that kids get, etc., but I didn’t make the system, and I’m not the only person who sees it this way.