I’m also a known cheerleader for NMH (my daughter’s school) but I think it fits your criteria, especially now that I see you are looking for progressive and non preppy - it stands out amongst its peers in both those categories and my daughter choose it specifically because we thought the kids were the most friendly. It has a very chill vibe, the kids are not competitive or cliquey and it is one of the most diverse schools too - 1/3 international roughly.