Purpose: One of the features of the New Zealand secondary schools system is that achievement closely reflects the taught curriculum. The National Certificate of Educational Achievement (NCEA) directly assesses student achievement on the secondary school curriculum through a combination of criterion-based internal and external assessments. The nature of NCEA means school-level results not only reflect student achievement but also the ability of leaders to organise, deliver, and monitor a relevant curriculum for students. This paper aims to describe how NCEA data were used to develop a simple but fair system to assess the relative performance of secondary schools. Design/methodology/approach: No standardised measures of performance prior to Year 11 are available in New Zealand. Nor are student-level data available. In the absence of these, multiple indicators of gross performance, added value and improvement over time were analysed using a schools-of-similar-type methodology. Findings: Results indicated that schools in the low and middle SES communities were more likely to be improving than others, but these improving schools were also more likely to be already high- or mid-performing. Low-performing schools were least likely to be improving. Originality/value: Some advantages of this methodology are its ability to be utilised with any publicly available standards-based achievement data, its validity as an indicator of leadership and organisational performance, and its ability to track school performance trends over time. (Contains 1 note, 3 tables, and 1 figure.)