Although educators frequently use assessment to identify who needs supplemental instruction and if that instruction is working, there is a lack of research investigating assessment that informs what instruction students need. The purpose of the current study was to determine if a brief (approximately 20 min) task that reflects a common middle school expectation (writing in response to text) provides educators with information about students' strengths and weaknesses in four research-based components of writing. Results indicated that, at the end of elementary school (Grade 5), students' word- and sentence-level errors, text-level plan, and typing fluency predicted 43% of their performance in written composition quality and all these factors play a role in writing achievement. At the end of middle school (Grade 8), text-level plan and word-level accuracy remained important components. Implications for using assessment to guide selection of evidence-based writing instruction throughout middle school are discussed. [This paper was published in "Journal of School Psychology" v94 p28-48 2022.]