This paper investigates Mandarin learning experiences of Chinese American teenagers from working-class families. Drawing on a subset of data from a larger ethnographic study, we focus on 14 middle schoolers who studied Mandarin as a heritage language at a socially engaging school with Mandarin as part of its official curriculum. The data highlight a mismatch between students' true heritage language and the institutionalized surrogate heritage language, which created unwanted implications for students' investment and identities in the presence of Mandarin hegemony. In a time when the teaching of Mandarin receives unprecedented educational support, this study calls for more educational efforts that attend to the large population of heritage language learners from non-Mandarin language backgrounds. We suggest that Mandarin educators have critical language awareness so they can address Chinese American students' multilingual backgrounds in ways that can value their home languages and support their investment in Mandarin.