I’m happy to help with my past experiences, but I think it really depends on what city you are looking for and what sport. Are you in the Boston area? If so, I can’t help there, but in NYC, it’s cutthroat for day schools and some of it is determined by money (by that I mean billions, not a million) and celebrity, along with being smart. I have extended family who chose to go the athletic route, but (as others mentioned above with regards to soccer), you know pretty early on. My family member went to Waterville Valley Academy at an early age and then continued on from there. I know families here in NYC who pulled their kids out of places like Spence (which is very hard to get into to begin with) to homeschool to pursue their sports. The only high caliber athlete I’ve heard of at a private NYC school is Lia Neal (swimming/CSH) although some of the catholic schools do well with rugby (Xavier). Blair has a nationally ranked wresting program, so one could be a day student there.