This paper offers a new way of understanding critical multicultural education through the lens of bi-directional assimilation (Alba & Nee, 2003), which better captures the connection between the pedagogical strategy of multiculturalism, and nation-building in a pluralist society. Bi-directional assimilation is a process of ethnic change experienced by "both" mainstream institutions (e.g., schools) "and" immigrant communities, whereby the ethnic distance between them is reduced. I argue that critical multicultural education can be reframed as a mechanism of bi-directional assimilation in schools, as it allows for ethnic change at the institutional and group level. In order to illustrate the relationship between bi-directional assimilation, multicultural education, and nation-making, I draw on findings from an ethnographic study of Castro Middle School -- a socially and culturally diverse school in California. My analysis shows how critical multicultural education can produce a model of Americanness that is more ethnically similar to minority students and more pluralist in nature. By contrast, hegemonic multiculturalism has the effect of distancing ethnic minority students from the national core group.