Understanding of the engineering design process was examined for mixed grade (9-12) high school introductory engineering classes. The classes consisted of videos on engineering, guest speakers, internet research on engineering careers, and hands-on design projects. Student interactions were analyzed with classroom observations, video recordings, and interviews and showed there was a significant effect of maturity on learning. Change in understanding of the design process was measured by an open-ended pre and post class test with a 40 point scale rubric. It evaluated solution generation and selection, design reports, teamwork, project management, and ethics. A pre-post t-test indicated a significant increase in understanding (p < .00). Students in grade 10 had the largest gain of 6.82 points, grade 12 the smallest with 1.14 points while grades 9 and 11 had moderate gains of 4.2 and 4.3 points, respectively. The limited gains were due, at least in part, to enrollment and student interaction issues in the mixed-grade, large enrollment classes. Recommendations for positive change are discussed.