Over the last decade, policymakers have been experimenting with competency-based education, an instructional reform that relies on flexible pacing to enable students to achieve content mastery at their own pace. In this paper, we draw on mixed-methods data from teacher surveys and interviews to examine the use of flexible instructional pacing in five Michigan school districts implementing competency-based education. While implementing flexible pacing was challenging for all five districts, we identified several promising practices that facilitated flexible pacing in their districts. These included the adoption of school-wide interventions and the ability of teachers to share students across classrooms. These practices resulted from explicit prioritization of flexible pacing in some districts, whereas in others, they occurred somewhat by happenstance. In all cases, structural challenges (e.g., the division of time and space and the allocation of students to individual classrooms) inherent in "the grammar of schooling" impeded some or all efforts to implement flexible pacing. It will be essential to tackle these structural challenges to flexible pacing in future efforts to implement competency-based education reforms.