This is the salient point in this discussion. Students who repeat ninth grade at the schools discussed on this board are not repeating material. They take assessments before entering and are placed in appropriate-level courses. They will graduate with the highest course levels they were able to consume from curriculums they were not able to outpace. Colleges do not see this practice at these schools in any negative way whatsoever; they know these schools intimately and are happy to accept their very well-prepared students. Remember, too, that these repeats are only competing against their peers, many of whom also repeated at that same school.
(I don’t know if @fretfulmother is aware that this forum is almost completely dedicated to boarding schools which may not compare well to the general pool of private schools.)