Clark was educated at Wixenford School and, from 1917 to 1922, Winchester College. The latter was known for its intellectual rigour and – to Clark's dismay – enthusiasm for sports, but it also encouraged its pupils to develop interests in the arts. The headmaster, Montague Rendall, was a devotee of Italian painting and sculpture, and inspired Clark, among many others, to appreciate the works of Giotto, Botticelli, Bellini and their compatriots. The school library contained the collected writings of John Ruskin, which Clark read avidly, and which influenced him for the rest of his life, not only in their artistic judgments but in their progressive political and social beliefs.