A school district and a teacher generally do not have any liability for giving an assignment a parent finds offensive. You should contact the teacher, the campus-level administration, the district-level administration, and the school board if you're concerned. They may allow you to exempt your child from the assignment and may remove the book from the curriculum altogether.
This is for informational purposes only and should not be considered as legal advice. This communication does not create an attorney-client relationship. Without knowing the specifics of your situation and prior to engaging in an attorney-client relationship I am unable to provide specific direction.