In this article, we provide empirical evidence on the utility of a teacher-mediated diagnostic classroom assessment intervention for identifying and addressing learning gaps in learner domains. Early results, based on 11 treatment and 27 comparison classes with 381 students and 38 teachers overall, indicated positive changes in teachers' assessment practices and attitudes, with corresponding student achievement gains. Supplementary observations pointed to challenges in creating a learning-centered assessment culture in schools operating under a high stakes assessment climate. We situated the findings in existing research evidence on formative classroom assessment, test-driven school reforms, and the currently available body of knowledge on how individuals learn, and argue for more proximally conducted formative assessment by teachers to address learner needs and achievement gaps in subject area domains. (Contains 2 figures and 6 tables.)