Built in 1905 and opened in 1906 as Naugatuck High School, now Hillside Intermediate School, exemplifies Neo-Classical Revival architecture. Designed by the renowned architectural firm McKim, Mead & White and commissioned by philanthropist John Howard Whittemore, the building was placed on a hill overlooking the Naugatuck Green. Its unique site allowed entrances on three levels, a feature recognized in Ripley’s Believe It or Not. Upon completion, the school was hailed as “America’s best equipped and most sumptuous public-school building.”

Whittemore’s vision for the town’s civic improvement led to the construction of Salem School in 1894, which initially served both as a grammar school and a high school. However, by 1900, it became clear that a new high school was needed. Whittemore hired McKim, Mead & White around 1901 to design a new facility.

Frederick F. Schaffer’s house sat in the spot that Mr. Whittemore wanted for the school's location. Estimated in 1904, he purchased the house from Mr. Schaffer, who then moved to a home on Rockwell Avenue. Mr. Whittemore then moved the house across the street to make room for Hillside School.

Greek and Roman Classical forms, consistent with the Beaux-Arts training of the architects, inspire the design of Hillside Intermediate School. Constructed of pressed buff brick and pink Milford granite, the school’s architecture resembles a Grecian temple. Triple flights of granite stairs lead to the south entrance beneath a two-story Ionic pavilion. The school’s east facade, visible from downtown, features a monumental Ionic colonnade and bronze anthemion decorations restored following a 1961 fire. The building's complex hipped roof, surrounded by copper trim and limestone trim, further emphasizes the classical style.

Designated as part of the Naugatuck Center Historic District on July 30, 1999.