Have you tried Westchester Equestrian? I haven’t been there in a lot of years so their staff might have changed a little bit. They weren’t always friendly, but they are one of the only dressage barns near NYC that have school horses.
I live in NYC and keep my horse at The Red Barn on Long Island (Old Brookville). They have a number of school horses and a couple trainers who are absolutely lovely but they have a hunter/jumper focus. I’m a dressage rider myself, and have trained a number of students through the levels – and have worked with students with disabilities of all kinds including learning, intellectual and physical. I’m not technically a trainer there, but we might be able to work something out.