Most evidence on early literacy tutoring comes from intensive one-year interventions. Much less is known about whether schools can sustain individualized instructional support as students’ reading needs evolve across the early grades. We present findings from a four-year randomized controlled trial evaluating Chapter One, an adaptive instructional support system designed to translate the instructional principles underlying effective tutoring into a scalable model for early literacy. We randomly assigned 815 kindergarten students in a large urban district in the Southeast to Chapter One or business-as-usual instruction and followed them through 3rd grade. Students assigned to Chapter One scored 0.12 to 0.17 standard deviations higher on state-administered reading assessments across grades 1 through 3, although the third-grade estimate is imprecise. They were also less likely to have an individualized education program (IEP) or be retained by the end of the study. The reduction in IEP status was concentrated among students who already had an IEP at baseline. The estimated longitudinal effects on reading are smaller than those typically reported for intensive tutoring models but were achieved under ordinary school staffing, scheduling, budget, and student mobility constraints. The findings suggest that schools can translate the instructional principles underlying effective tutoring into adaptive instructional support systems that improve students’ reading trajectories and academic progression while remaining feasible to sustain across the learning-to-read years.