Mainstream secondary school can be a challenging environment for autistic students, in part due to social and sensory factors. Research to date has focussed on identifying the negative aspects of school experience; however, few studies have identified factors that promote positive experiences. We take a neuro-affirmative approach when exploring how schools can support autistic students, informed by both the social and the human rights models of disability. We conducted a systematic review of qualitative studies in which autistic adolescents and adults identified factors that related to positive experiences in mainstream secondary school. We identified 117 relevant studies and 36 met inclusion criteria. Through thematic synthesis, we developed overarching themes: 'Understanding and Acceptance', 'Environments that suit me' and 'Agency to meet my needs', with additional subthemes. This study identified that agency, understanding and acceptance by both staff and peers, as well as access to engaging activities, were key factors in creating a positive and supportive environment for autistic students. Adaptation to both the social and sensory contexts were important to meet the needs of autistic students. The findings suggest that policy and practice should focus on creating a positive environment in schools for autistic students, in addition to addressing current challenges.