This longitudinal study of middle school science teachers explored the relationship, if any, between teacher participation in whole-school, sustained, collaborative professional development and student achievement in science. Eleven teachers from Glendale Middle School participated in the Discovery Model Schools Initiative 2-week summer institute, followed by monthly release day professional development sessions focused on implementing instruction outlined in the National Science Education Standards. Student achievement was assessed using the Discovery Inquiry Test in Science. The same students completed the test in grades 6–8. Students of teachers at Glendale Middle School significantly outperformed students at the control school. Findings in this study revealed the positive impact that whole-school, sustained, collaborative professional development programs have on student achievement, indicating that programs of this nature could be a means to narrowing or eliminating achievement gaps in science.