Imagine that you're the parent of 13-year-old twins. One thrives in school: His test scores place him in the top quartile of his peers. The other struggles: He's not motivated to learn what schools want to teach, and his test scores put him in the bottom quartile.
It's time to select high schools. You decide that the boys would benefit by going to separate schools, and you live in a place (such as Washington, D.C.) where they can choose schools through a lottery. You feel lucky when that lottery gives you slots in two schools - until you discover a disturbing truth.