With the death last week of Theodore R. Sizer, precollegiate education lost one of its most influential thinkers and a founder of the contemporary movement to improve schools. Mr. Sizer died Oct. 21 at his home in Harvard, Mass., of colon cancer. He was 77. Over his long career, Mr. Sizer was dean of the Harvard Graduate School of Education, headmaster of the Phillips Academy in Andover, Mass., founder of the Coalition of Essential Schools, chairman of the education department at Brown University, and founding director of the Annenberg Institute for School Reform. More recently, he helped start the Francis W. Parker Charter Essential School in Devens, Mass.