Proportional reasoning is the cornerstone of many higher-level mathematics topics. We evaluated the sustainability effects of schema-based instruction (SBI) on proportional reasoning and the impact of professional development (PD) on the instructional practices of two cohorts of seventh-grade teachers (i.e., SBI-experienced-teacher implementers, who taught SBI in a previous study; SBI-novice-teacher implementers, who taught SBI for the first time with PD) and mathematics outcomes of students with mathematics learning difficulties. SBI was implemented for 6 weeks by 49 teachers in two distal states. Despite the cessation of PD provided to SBI experienced-teacher implementers, teachers in both cohorts demonstrated similar levels of fidelity. Results indicated no SBI experience effect on the immediate and 11-week retention tests of proportional problem solving, on a general measure of problem solving, and on an end-of-year mathematics achievement test, suggesting that the effectiveness of SBI generalizes over time to different cohorts of teachers and students.