"G[superscript 2]ROW STEM" is an Investing in Innovation (i3) development grant funded by the Office of Innovation and Improvement, U.S. Department of Education. "G[superscript 2]ROW STEM" targets high-need middle school students, particularly females and minorities, that tend to be underrepresented in STEM careers. "G[superscript 2]ROW STEM" focuses on providing students with engaging, hands-on, project-based, extended-learning experiences to inspire interest in STEM and improve achievement. The impact evaluation used a quasi-experimental design (QED) to examine the effect of "G[superscript 2]ROW STEM" on academic achievement in science and math. "G[superscript 2]ROW STEM" student enrollment began during the 2016-17 school year and students were followed for three years, through the 2018-19 school year. Academic achievement outcomes for "G[superscript 2]ROW STEM" students were compared to a matched sample of students within the same grade in the same schools who participated in business-as-usual, traditional academic instruction with no extended learning experiences. Academic achievement for comparison students was also tracked over a three-year period. Results showed statistically significant differences between treatment and comparison groups in achievement test scores across both math and science. Overall, treatment students gained seven NCE score points more than the comparison group students did after up to three years in the "G[superscript 2]ROW STEM" program. This is roughly equivalent to one-third of a standard deviation. Significant differences were also noted for student subgroups with non-minority students realizing slightly higher gains in NCE scores than minority students by the end of the program (over all analyses a difference of 2.25 NCE points).