Engineering and integrated STEM experiences are being promoted at the K-12 level to increase interest and retention in STEM and to reinforce learning of mathematics and science content. However, research is still emerging regarding best practices for curriculum development, student impacts, and transfer of knowledge across disciplines. The purpose of this study is to investigate the impact of middle school engineering curriculum on students' academic achievement in science and mathematics and also on non-cognitive skills such as student engagement and self-efficacy in academics. Specifically, the Engineering Design Process (EDP) conceptual model is used as a framework for the engineering curriculum, which is also grounded in Problem-Based Learning (PBL) practices while integrating science practices and foundational mathematics. Participants include 6th-8th grade students at four public middle schools in Georgia. The research results show that students who have taken at least two engineering courses show statistically significant gains on state-level standardized science and mathematics tests over those students who were never enrolled in these courses. Further, the results show a statistically significant increase in cognitive and behavioral engagement in STEM and science interest. The results of this study support the idea that enabling students to practice their science and mathematics skills and knowledge within the context of interesting and engaging middle school engineering classes can significantly benefit both their engagement in STEM and their academic achievement.