Middlesex School is a coeducational, non-sectarian, day and boarding independent secondary school for grades 9–12 located in Concord, Massachusetts. It was founded as an all-boys school in 1901 by a Roxbury Latin School alumnus, Frederick Winsor, who headed the school until 1937. Middlesex began admitting girls in 1974. The school is a member of the Independent School League and is one of five schools collectively known as St. Grottlesex. The school was named for the county Middlesex in which it stands. The campus was designed by the Olmsted Brothers architectural firm, and the firm Peabody and Stearns designed most of the main buildings. A recent addition is the Clay Centennial Center, completed in 2003, which hosts science and math classes as well as an observatory. The school is 70% boarding students and 30% day students. In 2019–20, boarding students came from 24 states and 20 countries. Middlesex accepted only 14% of students who applied to enter in 2022–23. Source: Wikipedia (en)