Ratio and proportional relationships, along with the interrelated topics of fractions,decimals, and percent, provide a critical foundation for algebra (National Mathematics Advisory Panel, 2008). Proportional reasoning, which not only requires understanding the concept of ratios and that two or more ratios are equal, also requires the ability to extract relevant information to develop a representation of the problem situation and is challenging for many children and adolescents, especially students with mathematics difficulties (Özgün-Koca & Altay, 2009).The intervention investigated in this study represents an approach to proportional problem solving via schema-based instruction (SBI). Jitendra, Harwell, Dupuis, and Karl (2017a)examined the effectiveness of SBI for a subsample of students with mathematics difficulties (MD) selected from a randomized cluster study (Jitendra et al., 2015). The findings raise an important question: What is the range of proportional reasoning skills and general mathematical proficiencies for which SBI enhances student performance? The present study uses a regression discontinuity approach to identify the boundaries of the effectiveness of SBI for students with and without MD. Using a regression discontinuity approach provides evidence SBI is effective for a wide range of proportional reasoning skills and general mathematical proficiencies and thus broadens the population of students SBI can be used effectively with. Standardized slopes for the treatment effect also provide evidence the magnitude of the SBI effect is non-negligible. [SREE documents are structured abstracts of SREE conference symposium, panel, and paper or poster submissions.]