This thread had a woman lauding APS but attended Sidwell, as identified from her LinkedIn profile.
https://www.dcurbanmom.com/jforum/posts/list/15/1091696.page
I’ve seen Palo Alto High School, Deerfield Academy, NCS, all sorts of private high schools.
For the most part, you are admitted to college on your merits (I know that is up for debate) but the high school you attend is 99% the efforts of your parents (even a magnet, since they likely managed the logistics to make you eligible).
I understand if you are a fellow Sidwell alum, it engenders good will to the applicant, but as a poor person growing up it feels gauche to me. But maybe that’s how the world works, looking for people from your class at your IB, PE, law firm….?