The assessment of writing proficiency generally includes analyses of the specific linguistic and rhetorical features contained in the singular essays produced by students. However, researchers have recently proposed that an individual's ability to flexibly adapt the linguistic properties of their writing might more closely capture writing skill. However, the features of the task, learner, and educational context that influence this flexibility remain largely unknown. The current study extends this research by examining relations between linguistic flexibility, reading comprehension ability, and feedback in the context of an automated writing evaluation system. Students (n = 131) wrote and revised six essays in an automated writing evaluation system and were provided both summative and formative feedback on their writing. Additionally, half of the students had access to a spelling and grammar checker that provided lower-level feedback during the writing period. The results provide evidence for the fact that developing writers demonstrate linguistic flexibility across the essays that they produce. However, analyses also indicate that lower-level feedback (i.e., spelling and grammar feedback) have little to no impact on the properties of students' essays nor on their variability across prompts or drafts. Overall, the current study provides important insights into the role of flexibility in writing skill and develops a strong foundation on which to conduct future research and educational interventions. [This paper was published in: "LAK18," March 7-9, 2018, Sydney, NSW, Australia, ACM, 2018, pp. 380-388.]