Comprehensive sexuality education (CSE) is a key component of efforts to improve sexual and reproductive outcomes for young people. While many governments have established policies and curricula for CSE, there are no quantitative measures of the quality of their implementation in schools. We describe the construction of a school-level index to measure CSE implementation quality using data from Peru, Guatemala, Kenya and Ghana for validation. The composite index represents seven key components of a successful programme: range of topics, values imparted, teaching methods, teacher training, resources available, monitoring and evaluation, and the school environment. These components are combined into two sub-indices (curriculum content and implementation context), which together provide a summary index of implementation quality. We present scores for the seven components, the two sub-indices and the summary index for each country. Ghana scored highest on the summary index, while Guatemala scored lowest, but country rankings varied within the different components. The paper discusses the implications of the findings for improving school-based CSE in these countries. An assessment of the validity and reliability of the index indicates it can be used in other settings and could be a useful tool for educationalists, researchers and programme implementers to measure CSE implementation quality.