Last year, we visited and considered the Subbury Valley School in Framingham, Massachusetts (the original one). A friend's daughter goes there. What we learned is that there has been a shift in philosophy over the years. There used to be a role for adults as facilitators: they suggested activities that they thought the children would like and benefit from, and they helped bring them about. These days, there is a deep distrust of any idea coming from an adult. Even a suggestion is seen as compulsion. When children show interest in academic pursuits, it is often assumed that the parents have set them up to it. That is also seen as compulsion, and the activity is discouraged. As a result, the school is full of art and music, but the chemistry lab that was once there is gone, and the library is stale.
Instead, we ended up sending our daughter to Macomber Center for Self-Directed Learning, also in Framingham. It was started a couple of years ago as a response to what had happened at Sudbury Valley, and has kept the active role of the adults.