Although literacy achievement has improved in Chile, adolescents' underperformance in reading comprehension is still a serious concern. In English, core academic-language skills (CALS) have been found to significantly predict reading comprehension, even controlling for academic vocabulary knowledge. CALS are high-utility language skills that support reading comprehension across school content areas. Guided by an operational definition of Spanish CALS (S-CALS), three goals drove this study: to develop two psychometrically reliable tests, the S-CALS Instrument and the Spanish Academic Vocabulary (S-AVoc) Test; to explore the dimensionality of core academic-language proficiencies, as measured by these two tests; and to examine the contribution of core academic-language proficiencies to reading comprehension. A cross-sectional sample of 810 Chilean students (grades 4-8) participated in four assessments that measured standardized reading comprehension, word-reading fluency, Spanish academic vocabulary, and S-CALS. Using classical test theory and item response theory analyses, results yielded robust reliability evidence for both instruments. Consistent with prior research, S-CALS and academic vocabulary scores displayed upward trends in higher grades yet considerable within-grade variability. Confirmatory factor analysis revealed that S-CALS and S-AVoc were best conceptualized as part of a higher order construct, the Spanish core academic-language and vocabulary skills (S-CALVS). The aggregated S-CALVS scores predicted reading comprehension, beyond the contribution of grade, school factors, and word-reading fluency. This study advances our scientific understanding of CALS as relevant for adolescent literacy beyond the English language. The high-utility school-relevant language and vocabulary skills offer promising tools to inform and evaluate innovative reading comprehension interventions for Spanish-speaking adolescents.