Research on issues concerning the assessment of school-age English language learners (ELLs) in curriculum-learning contexts has been relatively less productive than assessment of adult language learners. A growing demand for assessing school-age ELLs has led to the development of assessment frameworks that provide the opportunity to examine the extent to which such frameworks represent the developmental continuum of these learners' language acquisition and literacy learning. Steps to English Proficiency (STEP) assessment framework were developed in Ontario, Canada, to be used by its teachers to assess, track, and support the language proficiency development of English language learners. STEP comprises three sets of descriptor-based developmental continua related to oral communication, reading, and writing, for each of four different grade clusters across Kindergarten to Grade 12. In this article we focus mainly on the characteristics of STEP language proficiency descriptors concerning: (a) the extent to which the descriptors are distinguishable across the skills of oral communication, reading, and writing; (b) the extent to which the STEP descriptors are distinguishable across the six proficiency levels; and (c) the extent to which the descriptors are homogeneous within each proficiency level. Our findings reveal that overall, the STEP scales are stable with six distinguishable STEPs and that there are strong correlational relationships among the three represented skills. Our research highlights the urgent need for research geared to understanding the relationship between oral and literacy skills for school-age language learners.