Widespread underachievement among students classified as English learners (ELs) indicates the United States education system is not serving them well. Although there has been increased attention directed toward the challenges these learners face in school, efforts to improve their academic outcomes often narrowly focus on English language abilities. Undoubtedly, English proficiency is a central component of academic achievement in the U.S. However, the emphasis on English language development in policy and practice also advances the idea that English is the only language for learning in school. Additionally, it obscures the contributions of non-linguistic influences on school success for language minority learners. Using mixed methods, this research investigates the school experiences of both high- and low-achieving Spanish-speaking ELs in Los Angeles Unified School District (LAUSD) with the aim of better understanding factors - other than English - that contribute to their differential academic trajectories. Growth-curve analyses of longitudinal, student-level data indicate that Spanish language abilities relate to English language arts (ELA) and math achievement in classrooms where the teaching and learning activities are conducted overwhelmingly, or exclusively, in English. Achievement in both ELA and math was higher as a function of increasing Spanish proficiency as measured in kindergarten, indicating that knowledge and skills gained at home in Spanish benefit learning endeavors at school in English. With ethnographic methods in a LAUSD middle school, the discussion of ELs' differential academic trajectories is extended to include social processes not detectable with the quantitative data. These qualitative findings suggest other factors that shape students' academic identities and create and maintain disparities in academic achievement. The practice of high-stakes achievement testing emerged as the primary school influence on the study participants' identities as learners. Through social identification processes the results of standardized achievement assessments become indicators of who students are as learners, as well as their learning potential, rather than simply scores on a test. The system of achievement assessments significantly affected access to curricula and quality instruction, and also influenced the relationships and interactions between students and teachers. Consequently, learning opportunities were expanded for some students and diminished for others.