I forgot to add - The Clark School limits their classrooms to 15 students. Kids work at their own pace and level. Acceleration is possible if a child completes the curriculum for their group and possesses the social and emotional maturity to do so.
One of the reasons why we decided on the Clark School was due to their ability to address a child's social and emotional needs as well as any special education issues like attention better than the Phoenix School or other private gifted schools in MA.
Both the Clark School and The Phoenix School have been around for 30 years and were started by teachers who knew one another. So there's a lot of common ground, though some differences as mentioned too.
I would suggest taking a tour and/or scheduling a visit for you and your child. That's one of the best ways to determine which school or setting may suit you and your child.