A teenager involved in a federal complaint accusing the DeSoto County School District of racial discrimination has been expelled, and the ACLU says it’s an act of retaliation.

The American Civil Liberties Union and its Mississippi chapter filed a lawsuit Monday in U.S. District Court on behalf of the ninth grader, identified only as A.S. The suit alleges a violation of the student’s constitutional rights.

The 15-year-old black student was suspended indefinitely from Olive Branch High School on the first day of class Aug. 10, after being accused of “throwing gang signs” during an assembly in the school gymnasium, the ACLU said. The lawsuit contends the student was quietly singing to himself while bopping his head and thumping his feet.

The lawsuit names the district, Olive Branch High School Principal Kyle Brigance, assistant principal Todd Nichols and the city of Olive Branch. It also names police Sgt. Toni Lesure and officer Doug Stanek, who are both school resource officers.

Kristy Bennett, legal director of the ACLU of Mississippi, said the student had never been suspended before the incident in the northern Mississippi district, just south of Memphis, Tenn. She also said the student has never been involved in gang activity, and school officials never claimed he was in a gang.

“To expel a high school freshman from school simply because he was singing to himself during an assembly is patently absurd,” Bennett said in a statement.

Bennett said the teen was targeted because he was one of several plaintiffs in a lawsuit filed against the district in April. That suit alleged six minority students were assaulted and discriminated against following an argument on an alternative school bus.

The suit was settled, but the terms were not disclosed. Four days after the settlement, A.S. was kicked out of school, Bennett said.

The school district’s attorney, Jay Keith Treadway, couldn’t be reached for comment. Police Chief Don Gammage didn’t immediately return calls from The Associated Press.

The lawsuit filed Monday contends that Stanek ordered A.S. off the gym bleachers and into the hallway to meet with Brigance, Nichols and Lesure. When the teen was taken back to Nichols’ office, the officers and school officials accused him of “throwing gang signs,” according to court documents.

On Aug. 14, disciplinary hearing officer Donald Corey recommended an indefinite suspension with a recommendation for expulsion. His mother appealed to the school board, but the panel never addressed the appeal, the lawsuit stated.

“The anti-gang policy here, which permits limitless and unfettered discretion to punish children for virtually any kind of conduct, constitutes a clear violation of students’ rights,” Catherine Y. Kim, staff attorney with the ACLU Racial Justice Program, said in a statement.

The suit is the third filed against the district by the ACLU this year. The organization in September sued on behalf of a student who was expelled after officials confiscated his cell phone and found what they described as “gang-related” images.