@ChoatieMom - you are right that I’m not very familiar with the boarding school forum on CC.
We may need to agree to disagree. All students who apply to college as HS seniors are competing based on a “high school record”. From what I have been told, elite colleges are unimpressed by someone taking five years to get whatever record they produce, and doubly so if this is hidden and then discovered.
Also, I fear that my post was misread - I did not say “4 years of math in 5 years” - I said “6 years of math in 5 years” - in other words, assume that you have a very good math student, who can move himself through the equivalent of a sixth year (e.g. Alg II, Geom, Precalc, Calc, Stats, DiffEq). This is more impressive to do in a regular four years of HS than if you get a reset 9th grade and do it in 5 years - even though both are “better” than a normal applicant might be.