The Common Core State Standards in Mathematics outline both the content and practices students must engage in at each grade level to become mathematically proficient. Mathematical processes include problem solving, reasoning and proof, communication, and procedural fluency, which includes flexible thinking. The purpose of this study was to evaluate the effectiveness of modified schema-based instruction (MSBI) on the acquisition and maintenance of math content and practices by middle school students with autism spectrum disorder (ASD). Two middle school students with ASD learned to solve proportional word problems containing extraneous information. Specifically, we measured mathematical problem-solving flexibility and communication using a 4-point rubric. Results of the reversal design found a functional relation between MSBI and the students' ability to flexibly solve the mathematical word problems and explain their answer, suggesting MSBI may be a useful strategy for some students with ASD.