How about letting the schools know you’d consider repeating, and let them decide? I know two kids who applied last year and got acceptances (to acronym schools) both for 9th and 10th grade. The admissions people are best suited to see the class composition and decide where you fit. And yes, if you go to 9th grade you will take most classes with other 9th graders, that is the point. You will get placement for math and foreign language. Starting with LA in 9th grade is OTT even for top boarding schools though, BC calc is the highest level class any 9th graders are in.