Answer: The first leader of the Notre-Dame School was Léonin, a Parisian composer and music theorist who lived in the 12th century. He is considered to be the first known composer of polyphonic music and is credited with pioneering the development of the Notre-Dame School, a style of music that dominated European classical music during the Medieval period. Léonin's contributions to music were significant and helped to lay the foundations for the development of Western classical music as we know it today.
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