Those schools expect that applicants will be “pretty much straight A” students (quote from a meeting at Hotchkiss). Based on sporadically published data, it appears that most students in those schools earned at least an 85% on SSATs, and for some it will be closer to 95%. They expect very good recommendations from teachers and others who know you well. They expect you to write good essays and interview well. The expect you to play a sport (and if you are very very good at it, that can be a strong plus). They expect you to have done something for your community.
However, it is worth noting that you can be all of those things above, and still not be accepted to any of those schools. That can be disheartening, but what it really comes down to is 1)if you want to go, you need to maximize your chances by doing all of these things and 2)you may want to consider why you want to go these very specific schools and learn about other schools that will also meet your needs, to enhance your chances of acceptance.
Kudos to you for starting a bit early and for having ambitious goals.