I don't see why we should have to teach diversity at all. Schools are for academic learning, not for imparting social values.
Children learn social values in school whether teachers consciously teach them or not. Both what is taught and what is not taught alters a child's perceptions of the world. Children who attend a school where the staff routinely neglects or dismisses diversity come away thinking that diversity is not important or that it is somehow bad. In a pluralistic society like ours, omitting the contributions of people from a variety of cultural groups tarnishes those groups and devalues their contributions. Teaching about diversity helps prepare children to live and work successfully in a pluralistic society.