The Union School is in the center of rural Hartford, Oxford County, Maine. The gable front vernacular schoolhouse with minimal Colonial Revival Style details is significant at the local level under Criterion A for its association with patterns of rural education and under Criterion C for its distinctive architectural characteristics of a rural one room schoolhouse. The building represents Hartford's response to evolving educational practices which were often mandated by state law. The school is an intermediary step between the often autonomous nineteenth century district system and the mid-twentieth century full consolidation of town and regional schools. The 1899 building was altered in 1923 to accommodate evolving trends in education. These alterations are significant as they occurred during the period of significance and reflect the evolution of educational best practices and the distinct characteristics of the building type. The period of significance is from the 1899 construction date to 1952 when the building was last used as a school. Of the many one room schoolhouses built in Hartford, several are still extant. These remaining school buildings have been greatly altered for use as homes or garages. There are currently no active schools in Hartford due to consolidation. The Union School is Hartford's best remaining example of the rural one-room school of the late nineteenth and early twentieth century.