Just how prestigious is the Millbrook School?

It's hard to say. It doesn't seem that private schools are ranked by Newsweek and U.S. News & World Report the way public high schools are. It's very small, with under 300 students, but I would guess that's the norm for private schools. It doesn't seem as well-known as, for example, Miss Porter's or Andover, but I don't think that's a reflection on its quality.