Reflecting concerns about student attainment and participation in mathematics and science, the Effecting Principled Improvement in STEM Education ("epiSTEMe") project attempted to support pedagogical advancement in these two disciplines. Using principles identified as effective in the research literature (and combining these in a novel fashion), the project developed topic modules for early secondary-school teaching in the UK, arranged for their implementation in classrooms, and evaluated the results. This article reports the development, implementation and evaluation of the "epiSTEMe" mathematics module entitled "Fractions, Ratios and Proportions." The module covers aspects of rational number and proportional reasoning relevant to the early secondary curriculum, and was developed in collaboration with teachers, implemented in 11 classrooms, and evaluated through comparison with 16 control classrooms where the topic was addressed using established methods. Students who used the "epiSTEMe" materials made significantly greater progress than control students as regards topic mastery, while holding positive opinions about their teaching and learning experiences.