Tom, Harry and Dick go to the principal's office..

Tom, Harry and Dick were misbehaving when they were playing outside in school so the teacher sends them to the principal's office.
Tom was the first to be called in to the office. The principal asks him "Why were you sent to my office?".
He replies with "I was being bad when I was playing in the sand pit with Harry".
The principal knew that the children were young and the incident was minor so he said "Okay, If you can spell sand I will let you off from going to detention"
Tom replies with "S-A-N-D" The principal smiles and says "Correct. You are free from going to detention".
Harry is the second to be called in to the office. The principal asks the same question to Harry about why he was sent to the office, to which he replies "I was misbehaving when I was playing in the sand pit with Tom"
The principal says "If you can spell pit I will let you off from detention"
Tom replies with "P-I-T" The principal gladly says "Well done, you are free from going to detention."
Finally Dick is called into the principal's office and the principal asks him "Why were you sent to my office?"
Dick replies with "I was watching Tom and Harry play in the sand pit"
The principal, puzzled, enquires "Why weren't you playing with them?"
Tom says "Because i'm black."
The principal then says "Alright, if you can spell racial discrimination I will free you from going to detention."