Many students come in at 10th. The school reserves many spots for 10th and expects the class to fully expand with 10th graders. Repeating 9th only makes sense if you need more time to get some basic writing and math skills acquired. I don’t think applying for 9th will give you any kind of advantage. In both years, AOs are looking for students who have leadership potential and who have unique and interesting stories. The grade you apply to won’t be a major factor in their decision.