Juliet Ryan’s 15-year-old son was celebrating his last day at school in Clacton-on-Sea in Essex on Friday, with students writing farewell messages on each other’s shirts.
But when he got home and his mother read the messages she noticed some racist slurs had been written in between the names.
Ms Ryan told Sky News: “He came through the door, he was bouncy and really happy it was his last day at school.
“I looked at the back of his shirt and underneath the neck area it had the N-word written on it.
“It had coon written on it twice and then it had ‘go back to the jungle’ written on it.
“He was distraught, he’s traumatised.
“I was quite upset because it’s racial hatred, it’s racial discrimination, it’s against the law.”
Ms Ryan, who is chief executive of equalities charity Working Action Group, said she was so upset she was crying, but she pulled herself together for a meeting with the headteacher.
He was “sincerely apologetic and said he was going to take it very seriously”.
She added that her son had suffered from racism before at the school but she felt those incidents were not taken “as seriously as I thought they should have been”.
“I don’t think they were dealing with the undercurrent and overt racism my son suffered from for almost three years,” she said.
“We feel like it’s going back in time, I faced severe racism when I was at school, also in Essex, and its affected me in more ways than one.”
With tears falling down her face, she said: “Racism needs to be eradicated and it’s not fair on my son. It hurts so much.”
Ms Ryan says she has reported the incident to the police, who have recorded it as a hate crime.
Sky News has contacted the school for a comment.