From elementary through high school, New York City children tend to go to school with others similar to themselves, in one of the country’s most racially segregated systems.

Turns out that racial segregation is an issue in prekindergarten, too.

A report by the Century Foundation, a public policy research group, which will be released on Tuesday, found that in 2014-15, the first year of the major prekindergarten expansion pushed by Mayor Bill de Blasio, a Democrat, prekindergarten classrooms tended to be more racially homogeneous than even the city’s public kindergartens.

In half of all prekindergarten classrooms, over 70 percent of students belonged to a single racial or ethnic group, despite the fact that the overall program was diverse, with no racial or ethnic majority. In one out of every six pre-K classrooms, more than 90 percent of the students were of the same race or ethnicity. In kindergarten, that is true in one out of every eight classrooms.

“As much as we struggle with segregation in K-12 schools, early education is really behind,” said Halley Potter, a fellow at the Century Foundation and the author of the report.