Although many adolescents struggle to comprehend text, the school-relevant language skills, which might contribute to variation in reading comprehension ability during this developmental period, remain understudied. To expand the research base, this study examines the concurrent development of academic language skills and reading comprehension in a sample of emergent bilinguals (EBs) and their English proficient (EP) peers (n = 573) followed over two academic years, from Grade six to seven. Parallel process latent growth modeling results reveal that academic language and reading comprehension skills develop concurrently, with growth rates in Core Academic Language Skills (CALS) positively associated with growth rates in reading comprehension. Furthermore, initial levels of CALS predicted rates of growth in reading comprehension. We also find that EBs, who are by definition in the process of acquiring English, begin Grade six with CALS that are far below their peers, but develop these skills at similar rates as their EP classmates. Results reveal the potential for CALS-focused instruction for improving adolescent learners' reading comprehension development, and highlight the particular relevance of supporting CALS for EBs, who are tasked with acquiring the language of print at the same time as they are acquiring other registers of English.