For years there has been mounting evidence that U.S. schools suspend and expel African-American students at higher rates than white students. A new study by the University of Pennsylvania singles out 13 Southern states where the problem is most dire.

Schools in these states were responsible for more than half of all suspensions and exclusions of black students nationwide.

"Black kids on the whole are suspended for reasons that have absolutely nothing to do with safety," says report co-author Shaun Harper of Penn's Center for the Study of Race and Equity in Education.

The 13 states named in the study are Alabama, Arkansas, Florida, Georgia, Kentucky, Louisiana, Mississippi, North Carolina, South Carolina, Tennessee, Texas, Virginia and West Virginia.

The researchers examined more than 3,000 school districts in those states. In 132 of those districts, they found, the suspension and expulsion rates of blacks were off the charts, with suspension rates far greater than their representation in the student body.

"Blacks are only 24 percent of students enrolled in public schools in those states, yet they are 48 percent of students suspended, 49 percent of students expelled," Harper says. "There are 84 districts where blacks were 100 percent of students suspended from school."

The new study is not the first to document such disparities. Other researchers have argued that schools use zero-tolerance discipline policies to, in effect, criminalize misdeeds such as dress code violations or talking back to a teacher.

The findings come as no surprise to critics of school discipline policies. Deborah Fowler is with Texas Appleseed, a public-interest law firm that conducted one of the most exhaustive studies of school suspensions and expulsions in Texas.

"In Texas, out-of-school suspensions have decreased by 20 percent over the last few years," she notes, "but as the numbers decrease, the disparities for black students increase."

In Virginia, state Superintendent Steve Staples said educators are working to address the problems.

"We agree the numbers are troubling," he says. Staples added that Virginia is tackling the problem through better training of teachers and administrators.

And, he added, those efforts are showing some results: "We've seen the short-term-suspension numbers drop, the long-term suspensions drop, the referrals to law enforcement drop."

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