China's rural-urban migration presents a significant educational challenge. This study uses theories of segmented assimilation and school segregation to measure the assimilation and well-being of migrant children who attend either Beijing's public schools or its informal migrant schools. Controlling for other factors, we find poorer achievement and greater loneliness among migrant children who are isolated in migrant schools than similar migrant students enrolled in regular urban public schools. We show there is little difference in learning outcome or loneliness between urban native children and migrant children who attend public schools. We further discuss similarities and differences between the experiences of migrant children in China and immigrant children in the United States. (Contains 5 tables, 1 figure, and 2 footnotes.)