Fixing the leaky pipeline: curriculum misalignment and STEM transition barriers in South Africa - a convergent mixed-methods study

South Africa's Science, Technology, Engineering, and Mathematics (STEM) education pipeline continues to face critical attrition, particularly during the transition from secondary schooling to post-secondary engineering pathways. This study examines how subject streaming, especially the widespread placement of learners into Mathematical Literacy, affects preparedness for both Technical and Vocational Education and Training (TVET) and university-level engineering studies. Employing a convergent mixed-methods design within an interpretivist paradigm, the research integrated structured surveys, semi-structured interviews, and focus group discussions. Purposive and stratified sampling enabled representation across rural, peri-urban, and urban contexts, engaging 120 qualitative participants (policymakers, principals, and STEM lecturers) and 180 quantitative respondents (secondary learners, STEM teachers, and tertiary students). Thematic analysis identified five systemic challenges: (1) curriculum misalignment as a structural barrier, (2) curriculum tracking and exclusion through Mathematical Literacy, (3) teacher capacity deficits in STEM pedagogy, (4) institutional fragmentation and policy incoherence, and (5) persistent urban-rural inequalities in STEM access. Quantitative data corroborated these findings, with 72% of teachers reporting insufficient training and 63% of tertiary students indicating low preparedness for engineering studies. The study recommends systemic reforms, including vertical curriculum articulation, bridging programs, and sustained STEM teacher development, to reduce attrition and strengthen South Africa's STEM-to-engineering pipeline for the Fourth Industrial Revolution.