A multi-State, quasi-experimental study was conducted as a longitudinal, two-year follow-up of participation in an online, on-demand professional development (PD) program. The purpose was to ascertain whether student gains were sustained in a second year of PD participation. Data verified gains in Year 1 versus Pre-PD baseline, with continued gains in Year 2 atop those achieved in year 1 of PD participation, reflecting a positive trend and continued advantage over non-PD schools in the same districts. Results showed that student in PD schools gained 7.7% (p<0.01) more in Math in year 2 atop 18.9% (p<0.001) gains from year 1, versus gains of 0.5% (ns) and 4.2% (p<0.01) for non-PD schools in the same districts. Similarly, students in the PD schools gained 10.2% (p<0.01) more in Reading in year 2 atop 18.9% (p<0.001) gains from year 1, versus gains of 0.5% (ns) and 4.2% (p<0.01) for non-PD schools in the same districts. Total gains from baseline for PD schools were 28.1% (p<0.001) in Math for PD schools versus 4.7% (p<0.01) for the same districts collectively, and 30.2% (p<0.001) in Reading for PD schools versus 6.0% (p<0.01) for the same districts collectively. Findings support the high-participation use of Internet-based, on-demand professional learning for improving teacher effectiveness and sustained impacts on student performance.