Linda Davidson/The Washington Post via Getty Images Gardendale High School biology teacher Justin Ingram readies his classroom before school begins in Gardendale, Alabama, on Aug. 4, 2016. Gardendale is a small city whose schools tried to secede from their larger district.

When Penn State University professor Erica Frankenberg graduated from high school in Alabama, there was only one school district in Mobile County.

Now, over 20 years later, it is one of four districts. In the past decade, three communities have splintered off to create their own districts, and, in doing so, they have exacerbated segregation in the area.

The process is called school district secession. Around the country, it’s changing the nature of school segregation.

A new study, conducted by Frankenberg, Virginia Commonwealth University professor Genevieve Siege-Hawley and researcher Kendra Taylor, looks at school secessions in the South with an eye on how new school district boundaries affect patterns of school and residential segregation. The study, which looks specifically at seven counties in the South where 18 new districts have formed since 2000, found that the practice increasingly sorts students into separate districts by race.

Thirty states allow for school district secessions, according to the nonprofit EdBuild. But only six are required to look at the socioeconomic and racial effects of these decisions. From 2000 to 2017, 47 communities across the country have successfully broken off from a larger district to form their own.

Communities often secede from large, integrated districts to create white enclaves in the name of neutral-sounding causes like “local control,” said Frankenberg. New districts tend to be whiter and more affluent than the ones they leave behind.

In the study’s seven districts, school district boundaries accounted for an average of about 60% of the school segregation of black and white students in 2000. But by 2015, this number had increased to about 70%. The remaining 30% can be attributed to school segregation within a district.