When a high school friend told me several years ago that he and his wife were leaving Washington's Mount Pleasant neighborhood for Montgomery County, I snickered and murmured something about white flight. Progressives who traveled regularly to Cuba and Brazil, they wanted better schools for their children. I saw their decision as one more example of liberal hypocrisy. I was childless then, but I have a 6-year-old now. And I know better. So to all the friends -- most but not all of them white -- whom I've chastised over the years for abandoning the District once their children reached...