preparatory school 
noun
Fuller form of prep school.School 1, a private preparatory school with some boarding pupils, had extensive facilities and 9.0 hours a week of physical education in the curriculum....
- The preparatory school for children aged seven to 13, which attracts boarders from all over the North of England, began taking younger day pupils five years ago, and demand has grown.
- Others have a different structure - there are, for example, senior schools, junior schools and preparatory schools.