Well I have two STEM kids and I would never send them to a school that wasn’t balanced in all respects. ( Both were SSAT 800 and 99% and had many STEM awards). Writing is important, so is history, art, foreign language, and all. What we looked at was the approach of the school. How does the school work with a kid who is ahead in Math/Science? Are there programs so the kids don’t’ run out of subjects? Are they WILLING to do individual classes of 1 or very small classes of similar kids. The schools my kids applied to fit the bill. Some better than others. One took my kid on a tour of the Science center and talked about some things which were unique to their program, others introduced my kids to the math chair, one asked very detailed questions during the interview about programming ( I had no idea), one talked about astro-photography as a possible art form. Some of these schools want outliers and others’ think that all of their accepted kids are really great and they can meet the needs of everyone. (Some can and some can’t).
We checked out Middlesex ( even applied) but it’s wasn’t a fit. And honestly, it wasn’t in our strong math/science category of schools. We did think the writing program was great ( though strange that it was offered in grade 10). My oldest didn’t really need writing help but found freshman English to be really challenging and the writing center at another BS was also excellent.
I think kids change a lot in BS. So you have to make sure they are strong in every subject. And the best way to check that is curriculum and how many kids they get that are outliers. They will tell you about kids who are similar to your own.