Mathematical problem solving has been identified as one foundational area of mathematics for all students but is an area of weakness for students with disabilities. This investigation sought to determine the effects of peer-delivered schema-based instruction on the mathematical problem solving of the change problem type with four middle school students with extensive support needs using a single-case multiple-probe-across-participants design. We also wanted to determine if same-age peer tutors who received behavior skills training could deliver the academic instruction with fidelity. Students and peer tutors were representative of diverse backgrounds. Results showed a functional relation between peer-delivered modified schema-based instruction and students' acquisition and maintenance of mathematical word problem--solving skills, and strong effect sizes were found. Peer tutors were able to provide instruction with high levels of fidelity across the study. Social validity data showed that all participants benefited from the peer-delivered instruction.