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 prestigious Independent School League and is one of five schools collectively known as St. Grottlesex.