English language proficiency (ELP) assessment scores are used by states to make high-stakes decisions related to linguistic support in instruction and assessment for English learner (EL) students and for EL student reclassification. Changes to both academic content standards and ELP academic standards within the last decade have resulted in increased academic rigor and language demands. In this study, we explored the association between EL student performance over time on content (English language arts and mathematics) and ELP assessments, generally finding evidence of positive associations. Modeling the simultaneous association between changes over time in both content and ELP assessment performance contributes empirical evidence about the role of language in ELA and mathematics development and provides contextual information to serve as validity evidence for score inferences for EL students.