If you feel strongly about sharing this with your class, I would not take the easy way out by removing the content from your lessons. If the principal has asked you to send an explanation home, I would simply state that you were trying to tie in real-life scenarios with your math lessons, since almost every child has asked at one time or another, "When will we ever have to use this outside of school?!" I would also say that you did not create the data and should not be blamed for the outcomes that the United States has produced, but that you will make sure to convey unbiased information and only allow facts to be taught.