"they send a lot of graduates to top schools"
That's just not true - - and you'll have a difficult time getting a straight answer about college matriculation. Like most second/third tier schools WNS posts an unannotated college list; the fact that Harvard, Yale, Amherst and Columbia appear on the list often the result of ONE student's acceptances - - not one student at each school, but one strong student who was accepted to all four. And since these lists usually cover a 5 year period, that means one student at Williams over the past 5 years. (The only place I've seen an annotated matricuolation list is in the annual report.)
I'm NOT suggesting that college placement should be the sole factor in school selection, but the placement stats do say something about the quality of the academic program, the academic prowess of the student body and the school's academic reputation. I don't recall the average SSAT, but I know that the SATs are generally average (in the 500-520 range; 2-4 NMS commended students per class, 0 NMS semi-finalists).
Also, in recent years, WNS has had difficulty retaining the stronger students from its middle school (a day school program for 7th and 8th graders). With the exception of 8th graders whose parents are WNS faculty, the strong middle-school grads enroll at top schools rather than stay at WNS, even as day students.
I have no doubt that the students are happy there, as a number of posts above suggest. But quality academics and happy students are not mutually exclusive, and if you want both WNS is not a particularly good choice b/c it doesn't really deliver on the academics.