Background: Learning to write the complex academic language (AL) associated with a discipline (like science) is a critical task in education, with middle school being a key developmental period. However, we need more research to guide how we assess students' learning to write AL, especially if we want to create assessment that guides more effective instruction. Aims: We evaluated middle school students' informational writing for six different measures of AL to determine which ones were most strongly related to writing quality and were most indicative of the unique features of informational writing. We also examined which metrics were sensitive to growth across middle school. Sample: Our sample consists of informational compositions from 285 students in Grade 5 (n = 175) and Grade 8 (n = 110) in a Midwestern state in the United States. Methods: Path modelling was used to determine the degree to which the AL metrics are associated with writing quality and narrativity in each grade. Results: Overall, the six measures of AL explained 70% of the variance in students' writing quality. We found that a new measure, number of long words, outperformed other more established measures at the word level and should be used in assessment of informational writing quality. We also found that automated scores at the sentence level better detected development across middle school grade levels than typical rubrics of writing quality. Conclusion: Results provide promising avenues for the assessment of malleable aspects of AL in middle school informational writing.