I know this is late, but I would really REALLY like to recommend Fay! Upper School students (grades 7-9) are offered math levels up to Algebra 2, and even pre-cal if a student has demonstrated special knowledge/ability. The math department is definitely great at recognizing potential in students, and all the teachers are super supportive by supplying extra or alternative work. I was moved up to Algebra I in seventh grade (from pre-alg), and I remember my teacher allowing me and two other students to move to a separate classroom once in a while to work on Exeter math problems when work wasn’t challenging enough. Another teacher helped me sign up for brilliant.com and Khan Academy. The science program is great too for those who make the most of it, and there’s a competitive robotics club and something else that has to do with a space simulation game.
To your concerns about SSAT math, Algebra really is a significant part of the test, and even a little geometry helps a lot. Fay also offers SSAT prep on weekends for 8th and 9th graders.
Though many students come in for the 7th grade, most of the boarders that stay through the 9th grade program (100% would recommend, by the way) arrive at 8th (the others opting to leave early for a 4-year high school experience). Many are on significant FA, if that’s of concern. Also, the secondary school placement office is AWESOME and matriculation stats are great. As a grad as of this past spring, I can tell you it was the best experience of my life.