A critical issue facing U.S. schools and one with broad social implication is the persistent disparity in educational achievement between racial/ethnic groups. The achievement gap may be particularly pronounced for Mexican-Americans who constitute the vast majority of U.S. Latinos and are among the most educationally at risk of all Latino subgroups. By employing mixed-methods research techniques, this study shows that social network instability accompanying high mobility rates may contribute to Mexican-American underachievement. Moreover, this investigation challenges wholly beneficial and ecumenical notions of social capital. Mexican-origin youth in possession of what on the surface appears to be a valued form of social currency may actually be the unwitting recipients of a form of counterfeit social capital that impinges on their school success. Efforts to reduce unnecessary and reactive student mobility and increase the stock of beneficial forms of social capital (while rooting out its impostors) may deserve policy consideration. (Contains 3 tables, 2 figures and 25 notes.)