Calls to support inclusive educational experiences for students with intellectual and developmental disabilities (IDD) have been longstanding. General education STEM classes may provide a rich context for promoting the inclusion of these students within a relevant curricular area. To assess inclusive class participation, we directly observed 15 secondary students with IDD--along with a comparison group of their classmates without disabilities--in STEM-related classes. We focused on academic, social, and contextual measures. Although some similarities were found in the academic and social participation of students and their classmates, key differences were observed in the areas of what they learned, who they conversed with, and how they learned. We present recommendations for future research and practice aimed at strengthening inclusive educational experiences.