Beyond academic vocabulary, the constellation of skills that comprise academic language proficiency has remained imprecisely defined. This study proposes an expanded operationalization of this construct referred to as core academic language skills (CALS). CALS refers to the knowledge and deployment of a repertoire of language forms and functions that co-occur with school learning tasks across disciplines. Using an innovative instrument, we explored CALS in a cross-sectional sample of 235 students in Grades 4–8. The results revealed between- and within-grade variability in CALS. Psychometric analyses yielded strong reliability and supported the presence of a single CALS factor, which was found to be predictive of reading comprehension. Our findings suggest that the CALS construct and instrument appear promising for exploring students’ school-relevant language skills.