This research reports results from a mixed methods study examining the role of adolescent English Language Learners’ (ELLs’) self-efficacy beliefs in the native language skills, and acculturation, on their achievement in their English for Speakers of Other Languages (ESOL) classes. Twenty-nine native speakers of Spanish enrolled in early levels of ESL classes were recruited from a large high school. Students were administered three surveys in small groups and were interviewed individually regarding educational experiences in their home countries and in the U.S. The results showed that student’s self-efficacy beliefs to perform well in their own native language significantly predicts student performance in English language. Acculturation also predicted achievement. Qualitative findings provided important information about learning experiences and contributed toward an updated definition of what types of schooling and learning experience some of our older adolescent immigrant students have had. These results may be useful in designing effective interventions for adolescents with interrupted schooling and suggest that heritage language classes in students’ L1 may contribute to greater academic success in L2 schooling.