In a nationwide initiative to increase computer science Education in K-12, many teachers were recruited to CS teaching positions but without adequate preparation, which can lead to lack of confidence and feelings of isolation [35, 54]. In response to these issues, the purpose of this study was to determine the effectiveness of a newly developed summer professional development program with follow-up support with the goal of improving teacher confidence and informing teacher beliefs about computer science. This training featured peer instruction and an emphasis on computer science principles. It took place over three years, and was used to prepare teachers from over 50 schools across a midwestern state. Using Hierarchical Linear Modeling, data analysis from 57 teachers and 871 students showed that teacher confidence and content knowledge increased significantly, and to a greater extent for those who began with the lowest levels of confidence. Students also made significant knowledge gains from before to after taking the class. Student gains were higher for those whose teachers were more confident in their ability to teach computer science.