This randomized controlled study investigated the efficacy of a Tier 1 intervention, schema-based instruction (SBI), designed to help students with and without mathematics difficulties (MD) develop proportional reasoning. Twenty seventh-grade teachers/classrooms were randomly assigned to a treatment (SBI) or control (business-as-usual) condition. Participants included 373 students of whom 253 demonstrated MD. A measure of proportional problem solving (PPS) was administered at pre- and post-testing, and at 11 weeks following treatment, along with a general mathematical problem solving measure at pre- and post-testing. 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. [This paper was published in "Exceptional Children" (EJ1151254) and "Journal of Learning Disabilities" v83 n4 p340-358 2017.]