It depends on what you are looking to accomplish. All three options are good ones. Darrow is a small school that is almost completely boarding and will provide close attention especially for a slow child. Lyndon has a very large day population and the boarding is almost all international, however, they have a very large number of classes to choose from. A small day school is a nice, but you never know what the host family will be like, they are also less likely to have much experience with international day students.