In 1997, Shields left his career in education to write independently. Over the course of the next six years, he published 20 histories and biographies for young people. In 2002, E.D. Hirsch (Cultural Literacy: What Every American Needs to Know, 1988) invited Shields to join his Core Knowledge Foundation in Charlottesville, Virginia as senior editor, contributing to a curriculum which, adapted, became the Common Core Standards Initiative that "define the knowledge and skills students should gain throughout their K-12 education in order to graduate high school prepared to succeed in entry-level careers, introductory academic college courses, and workforce training programs." Forty-one states, the District of Columbia, four territories, and the Department of Defense Education Activity have voluntarily adopted the Common Core.
