Second-generation immigrants are typically analyzed under the assumption that, having been born in the United States, they grew up in the United States. We challenge this assumption by investigating the prevalence and patterns of second-generation Mexican-American children's migration to and return from Mexico during childhood, and consider the consequences of this migration for their schooling. Around 1 in 10 second-generation Mexican-American children live in Mexico for some of their childhood. Strong patterns of return to the U.S. throughout childhood, and especially in early adulthood, argue for their being considered as part of the Mexican-American second generation even when growing up in Mexico. The school enrollment of these emigrating children in Mexico is much lower than for those second-generation Mexican-American children remaining in the U.S. through childhood. The moderately negative selectivity of emigrating second-generation children explains little of their much lower school enrollment. We conclude that country of residence is a far more important determinant than either family background or migrant status. (Contains 4 tables and 4 figures.) [This paper is part of the RAND Labor and Population working paper series. It was made possible by the NIA funded RAND Center for the Study of Aging and the NICHD funded RAND Population Research Center. An earlier draft of this paper was presented at the 2007 meeting of the Population Association of America.]