Autistic students often struggle to engage with peers in integrated education; however, research has largely focused on individual characteristics rather than the interpersonal and environmental factors affecting peer engagement. This mixed-methods study examined longitudinal peer interactions over a school year among 17 adolescents (seven were autistic) in an inclusive school club. The quantitative phase investigated participants' social behavior rates to identify sessions where each student demonstrated high and low peer engagement compared with their average participation levels. The qualitative phase compared social interactions and contexts between sessions of high and low peer engagement, revealing four themes regarding contextual supports and barriers to autistic peer engagement: (1) peer engagement is a participatory process where a student and their peer(s) navigate mutual understanding, shaped by both student and peer social characteristics, openness, and involvement; (2) student-peer synchronicity, such as shared interests or compatibility of social styles, was essential to autistic peer engagement; (3) peer engagement can be supported by activities facilitating joint engagement and exploration of mutual interests; (4) classroom interventions emphasizing strengths can support peer engagement, while normative behavioral standards without peer education on individual differences and diversity can perpetuate peers' negative perceptions of autistic difficulties.