Theodore Ryland Sizer (June 23, 1932 – October 21, 2009) was a leader of educational reform in the United States, the founder (and eventually President Emeritus) of the Essential school movement and was known for challenging longstanding practices and assumptions about the functioning of American secondary schools. Beginning in the late 1970s, he had worked with hundreds of high schools, studying the development and design of the American educational system, leading to his major work Horace's Compromise in 1984. In the same year, he founded the Coalition of Essential Schools based on the principles espoused in Horace's Compromise.
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