The school's conception dates to a few years before 1918 when Riddle purchased 3,000 acres (1,200 ha) of land on which to build it.

"[3] In 1918 she created the Pope-Brooks Foundation, to manage both her house, Hill-Stead and its artworks, and the as-yet unformed new school.

[3] For her designs Riddle was elected a Fellow of the American Institute of Architects, and awarded the Robinson Memorial Medal of the Architectural Club of New Haven.

[3] John Wallace Riddle set about designing the Avon buildings in a manner resembling the houses and spatial dimensions of an old English village.

A handbook published in 1925 said that Avon, then in planning, sought to cultivate "the sturdiness of character found in the old New England stock of Colonial times."

[3] Starting in June 1944, during World War II, the campus was adapted to serve as the Old Farms Convalescent Hospital for blind veterans.