I believe that 99% of teachers can effectively decide what is appropriate and what is not.  In the case that material may be sensitive, alternative assignments can be given.  I have not met a teacher yet who has not worked with parents and students to agree to alternatives.  This could result in the opposite end, where materials in schools are suffocated by trying to appeal to every sensitivity.  Principals should encourage teachers to come to them if they question their subject matter and have an open discussion, but there does not need to be legislation to state this.