The Japanese lesson study (JLS) model for curriculum development has seen limited application to middle school science classrooms. The JLS model was used to develop and refine three hands-on activities focused on four major eighth grade science topics from the California curriculum. Prior assessments of these topics showed limited understanding by students at a suburban school with a predominanty underrepresented population in science. Quantitative assessment of student understanding along with observations of the students' capacity to conduct careful investigations found a large, gender-independent increase in understanding, as well as improved state-mandated test scores. Informal "local proof" assessment resulted in expansion of the JLS model into other grade levels and disciplinary subjects. This led to multiyear school-site reform and recognition outcomes.