The district is Delaware's largest, serving 17,000 students in Newark and part of Wilmington.
School officials say the district is one of five nationwide whose discipline rates are being investigated by the U.S. Department of Education.
Schools spokeswoman Wendy Lapham says district officials acknowledge discipline rates differ for black and white students and they are cooperating with the investigation.
Statewide, about a third of public school students were black last year, and they accounted for more than half of those suspended or expelled, according to an analysis by The Wilmington News Journal.
Information from: The News Journal of Wilmington, Del., http://www.delawareonline.com