Population mobility is at an all-time high in human history. One result of this unprecedented movement of peoples around the world is that in many school systems monolingual and monocultural students are the exception rather than the rule, particularly in urban areas. This shift in demographic realities entails enormous challenges for educators and policy-makers. What do teachers need to know in order to teach effectively in linguistically and culturally diverse contexts? How long does it take second language learners to acquire proficiency in the language of school instruction? What are the...