A growing number of studies have shown the benefits of K-12 integrated science and engineering education. With this study, we add to the literature by documenting the relationship between STEM learning and engagement, and the demographic characteristics that impact achievement in STEM. This longitudinal study followed a diverse group of 245 middle school students from sixth grade to eighth grade. Students in two cohorts, cohort I and cohort II, participated in three different integrated STEM units during middle school, one in each grade level. Each curriculum unit focused on the same engineering concepts and practices but addressed different science content standards. The students completed science content tests and engagement surveys before and after the curriculum units. The results of the mixed-effects model analysis showed that behavioral engagement and career inspiration played a critical role in student achievement. While the achievement gap between students of color (SOC) and their White peers was narrowed in cohort II, the achievement gap between the SOC and their White peers in cohort I remained. Our analysis also revealed no gender differences in student achievement. These findings can inform the design of curriculum interventions that specifically target closing the achievement gaps between different groups of students.