This study addresses a current research gap in Medicine concerning Impact of School-Based Mental Health Support on Psychological Well-Being Among Primary School Students in Lagos, Nigeria: Longitudinal Psychometric Studies in Nigeria. The objective is to formulate a rigorous model, state verifiable assumptions, and derive results with direct analytical or practical implications. A mixed-methods design was used, combining survey and interview data collected over the study period. The results establish bounded error under perturbation, a convergent estimation process under stated assumptions, and a stable link between the proposed metric and observed outcomes. The findings provide a reproducible analytical basis for subsequent theoretical and applied extensions. Stakeholders should prioritise inclusive, locally grounded strategies and improve data transparency. Impact of School-Based Mental Health Support on Psychological Well-Being Among Primary School Students in Lagos, Nigeria: Longitudinal Psychometric Studies, Nigeria, Africa, Medicine, original research This work contributes a formal specification, transparent assumptions, and mathematically interpretable claims. Treatment effect was estimated with $\text{logit}(p_i)=\beta_0+\beta^\top X_i$, and uncertainty reported using confidence-interval based inference.