A conservative think tank led by former Department of Education officials has filed a federal civil rights complaint against an Illinois high school that announced it was implementing a grading system meant to equalize student outcomes by race.

In a Wednesday letter to the U.S. Department of Education, the president of the Defense of Freedom Institute Robert Eitel requested that the department’s Office for Civil Rights open an investigation into Oak Park and River Forest school district near Chicago after it began instituting “equitable” grading practices at a district high school.