Really, it depends on the school. There are quite a few that expand the class significantly sophomore year. (Our did. ) Many kids who want to go to BS are already there. So there are spots – fewer-- and fewer applicants. BUT… It is typically a different pool. JBS kids. Lots of international kids where this, not 9th, was the natural break and place to change schools. Kids who didn’t get in last year. Kids who realized their local option wasn’t a good one.
All you can do is put your best foot forward. Definitely ask the AO about the odds. If you are willing, ask about the benefits of repeating 9th.