Our experience at SPS echoes Goatmama’s comments. A lot of those older kids were late starters in K or 1st. To me, that should be viewed differently than repeating 9th, especially for sports. The late K/1st start was also quite commonplace at our local public schools.
The stats I posted in #6 were purely based on my own knowledge of students who truly repeated 9th grade when starting boarding school.