The qualifications for each private boarding school depend on the school. Often, as long as you can afford the tuition, you can attend the school. Sometimes, however, you will be required to pass certain placement exams or, if the school is focused on art, writing, or some other medium, you will need to prove proficiency in the medium. It really just depends on the school though, so you will need to check with the schools you are interested in to see what their requirements are.