In the early 1920’s, Mrs. Jennings, trustee, spearheaded the building project for the new one story Georgian-style revival brick schoolhouse. The new school would be located on the same site as the second schoolhouse. In November 1924, sealed bids were opened by the school board. The architectural firm Peabody, Wilson and Brown was selected. Construction began early in 1925 and was completed by the fall. The floor plan consisted of three classrooms, an auditorium and stage, and a library. Opening day was Monday, September 28th. When the fourth East Side School was built on Goose Hill Road in 1952, then the third school was usually referred to as “Main Street School” or “the Old School”.

In 1950, the East Side School kindergarten students attended classes in the Cold Spring Harbor Library-(presently Society for the Preservation of Long Island Antiquities-SPLIA Gallery) due to increasing enrollment and overcrowding. It was apparent that a new school building was needed. As a result, the fourth East Side School was built and opened in 1952. Extra space then became available in the third East Side building. The East Side School Trustees rented the classrooms to various organizations from 1954 through 1987. From 1962 through December 1985 the Cold Spring Harbor Central School District office was located here. From June 1987 to December 15, 1992, the Dolan DNA Learning Center leased the building and exercised their option to buy the building in the early part of 1993.