Grounded in Hallidayan perspectives on academic language, we report on our development of an educative science assessment as one component of the language-rich inquiry science for English-language learners teacher professional learning project for middle school science teachers. The project emphasizes the role of content-area writing to support teachers in diagnosing their students' emergent understandings of science inquiry practices, science content knowledge, and the academic language of science, with a particular focus on the needs of English-language learners. In our current school policy context, writing for meaningful purposes has received decreased attention as teachers struggle to cover large numbers of discrete content standards. Additionally, high-stakes assessments presented in multiple-choice format have become the definitive measure of student science learning, further de-emphasizing the value of academic writing for developing and expressing understanding. To counter these trends, we examine the implementation of educative assessment materials--writing-rich assessments designed to support teachers' instructional decision making. We report on the qualities of our educative assessment that supported teachers in diagnosing their students' emergent understandings, and how teacher-researcher collaborative scoring sessions and interpretation of assessment results led to changes in teachers' instructional decision making to better support students in expressing their scientific understandings. We conclude with implications of this work for theory, research, and practice.