This study investigated how autistic pupils' psychological, social, and educational outcomes differed in contrasting mainstream provisions. Using a three-year longitudinal quasi-experimental design, outcomes were compared across three placements: placement in a specialist resource centre (SRC), mainstream placement in SRC host schools (M-SRC), and placement in mainstream schools without an SRC (N-SRC). Autistic pupils (N=119, aged 11–14) and matched non-autistic peers (N=119) completed surveys and schools provided attainment, attendance, and exclusion data. Placement type showed no main effects on psychosocial outcomes in mixed ANOVAs, indicating limited evidence for differential change by placement alone, but hierarchical regressions indicated that SRC placement was statistically associated with higher academic attainment and stronger school belonging than N-SRC, and greater perceived teacher support than M-SRC. Attendance disparities for SRC pupils were smaller than national averages for autistic pupils, though attendance remained below whole-school averages. Perceived peer and teacher support were most strongly associated with positive outcomes across models, highlighting relational inclusion as a potential mechanism. Findings underscore the value of positive relationships in fostering belonging, reducing disparities, and promoting wellbeing for autistic pupils. Overall, SRC approaches may be associated with academic and belonging advantages, while relational inclusion appears central to positive outcomes.