Policy commitment to school-based sexuality education does not guarantee inclusive classroom enactment. This comparative critical policy and evidence synthesis examines how sexuality education commitments are implemented in South Africa and Sweden, two rights-oriented but structurally different systems. South Africa formally locates comprehensive sexuality education (CSE) within Life Skills and Life Orientation and has developed national HIV, STI, and TB policy commitments and scripted lesson plans to support implementation. Sweden has a longer history of compulsory school sexuality education and, following curricular amendments implemented in 2022, uses the policy terminology sexuality, consent and relationships (SCR), with responsibility distributed across subjects and school actors. Guided by critical interpretive synthesis, policy triangle analysis and implementation scholarship, we conducted a structured identification and verification process across official policy repositories, monitoring reports, Swepub, Libris, Libsearch, publisher records, PubMed/PMC, Google Scholar and DOI records. Evidence was eligible when it addressed school-based policy content, educator practice, learner experience, inclusion of marginalised learners, teacher education, curriculum resources, inspection/monitoring or implementation support in either country. The synthesis indicates that South Africa and Sweden share an implementation problem but not the same implementation mechanism. South Africa illustrates implementation fragility: a legitimate CSE mandate is repeatedly destabilised before inclusive enactment becomes routine through uneven educator preparation, moral contestation, resource constraints, school-level variability and selective silencing of sexual and gender diversity. Sweden illustrates implementation drift: an institutionalised SCR architecture may lose equity depth through subject-dispersed responsibility, assumptions of progressiveness, variable teacher education, uneven learner participation and persistent silences around culture, religion, racism, disability, intersex, and asexuality. The concepts of fragility and drift extend existing language of implementation gaps by identifying distinct mechanisms through which policy commitments become weakened. Both contexts require sustained teacher education, stronger school-level coordination, meaningful youth participation, marginalised-learner monitoring, inclusive-resource audits and accountability indicators that assess what learners actually experience in classrooms.