This article reports on the implementation of a formative assessment tool (the Writing Engagement Scale, or WES) in grades 3–5 in schools in the United States. We used confirmatory factor analysis (CFA) to collect validity evidence for the WES for our population. Results demonstrated acceptable validity and reliability. In addition, survey results indicated that teachers perceived the WES to be useful as a formative writing assessment. We make the case that the WES provides an opportunity to inform teachers’ practice and help researchers understand the dimensions of students’ engagement in writing.