As U.S. public schools have grown increasingly segregated by race and income, there is a growing number of school districts and charter schools striving for greater balance among their students, according to new research released Tuesday by the Century Foundation, a left-leaning think tank.

Researchers identified 91 school districts and charter school chains serving more than 4 million students — including the District of Columbia and Chicago Public Schools — that are using tools such as magnet schools, weighted lotteries, and changes in school attendance zones to create more balance between white students and those of color, and between low-income and more affluent children.