My son goes to a progressive Montessori school as well. Schools that had a friendly “Montessori” feel that we have visited have been Vermont Academy, Kimball Union and New Hampton. The Gunnery, Brewster, Putney, Lawrance Academy, Concord, Pomfret and Middlesex have all commented on how Montessori kids do really well in their programs. I guess they put their money where their mouths are because they all seem to have Montessori schools nearby where many staff send their children. All of those schools are close enough that you could easily visit a bunch of them in one visit.