Are there any federal laws regarding racial slurs and harassment in public schools?

My son has been harassed continuously by numerous kids and called the "N" word. The school won't do anything unless there are witnesses, and when that happens rumors get spread around the school and he's harassed more. One kid only got punished with 1 day suspension and continues to call him the N word but now he does it when my son and him are in the hallway alone. The school told me they can't do anything because there are no witnesses. I am infuriated. Are there federal laws regarding racial slurs and harassment in public schools that I can bring to the school board? I am looking on my own I haven't been able to find anything yet.