Richard D. Kahlenberg, senior fellow at the Century Foundation, writes: How should educators think about affirmative action in the age of Donald Trump? On the one hand, President Trump’s polarizing rhetoric underlines why it is so important to provide educational settings that bring students of different backgrounds together to learn to understand and respect one another. On the other hand, Trump’s rise suggests that the old ways of creating diversity — using large racial preferences that tend to benefit wealthier students of color — come at a considerable price for the nation.