This randomized controlled study investigated the efficacy of a Tier 1 intervention— schema-based instruction—designed to help students with and without mathematics difficulties (MD) develop proportional reasoning. Twenty seventh-grade teachers/classrooms were randomly assigned to a treatment condition (schema-based instruction) or control condition (business as usual). Participants included 373 students, of whom 253 demonstrated MD. A measure of proportional problem solving (PPS) was administered at pre- and posttesting and at 11 weeks following treatment, along with a general mathematical problem-solving measure at pre- and posttesting. For the full sample, posttest differences favoring the treatment group were statistically significant for all measures. For students with MD, posttest differences favored the treatment group for the PPS posttest and PPS delayed posttest but not for general problem-solving posttest.