You might want to look at Junior Boarding School. Look at Eaglebrook, for example. Most students go to BS from there for 10th grade although some will repeat 9th.
A couple of schools with small boarding communities that have middle schools are Pennington and Hun. Not sure at what grade boarding begins.
I personally would recommend the JBS route as it will give you the most flexibility. The problem with picking BS with 8th grade is that you narrow the universe so much. JBS tend to have great placement offices and excellent relationships with BS. Very helpful with fit!