Founded in 1870 by the Purchase Meeting of the Quakers, the Chappaqua Mountain Institute was a nonsectarian, coeducational, private academy providing a four-year educational program roughly equivalent to high school. Most of the students were boarders. Later, the Institute added elementary school classes. In New Castle, private academies provided students with their only option for an education beyond the common-school level until the early 20th century. The first school building, which was built mainly of wood, burned in February 1885. The second building, constructed of stone, brick and concrete, was completed by the end of 1886 and remained in operation until 1908. The former administrators and faculty gathered for a reunion on the front steps in June 1909.
After the school closed, the property was sold to the Children’s Aid Society of New York City, and was used as a convalescent hospital, mainly for children from New York City. The Elizabeth Milbank Anderson Home was closed in 1967. After the closing, fire destroyed the interior of the building, and the ruins were demolished. Beginning in 1956, the Children’s Aid Society used the grounds for Wagon Road Camp, a summer camp for children with illnesses and disabilities. It is now a day camp for children in the area.