I agree w previous posters about emphasizing willingness to get involved in new things. Depending on your personal situation you could discuss lack of such opportunities at your current school or alternatively the difficulty in outside activities due to cost or transportation or time!
My older son who was a mid HS transfer discussed what a poor fit his former school was (someplace he’d been since K) and that as a result he didn’t want to engage any more than necessary, which was why he wanted to transfer. (At his boarding school he tried all sorts of new things!)
As for repeating 9th grade - I think you need a substantive reason for doing so and not just because some people think it’s an easier entry point. You may feel you aren’t as academically prepared for 10th based on current school or you’re an athlete or you’re young for your peer group. A lesser 4th reason could be that you really want the full 4-year experience at boarding school. I’m a big fan of re-classing but I don’t think it makes admission easier.