Saudi Arabia has expanded access to secondary schooling over the past generation, while also pushing to lift quality. This includes decentralising authority out to principals, equipping them to set higher performance expectations and deploy incentives to attract and retain strong teachers. We find wide variability in the extent to which principals display leadership behaviours, based on nationally representative school samples drawn in 2003, 2011, and 2015 for the Third International Mathematics and Science Study. Pupil achievement did improve across these cohorts, markedly among girls attending schools in which principals enriched the academic climate and deployed teacher incentives, after taking into account the social class background of students and the school's instructional resources. Greater availability of resources, including computing tools, improved over the period but did not account for higher achievement in mathematics. Lessons for other nations are discussed, as international donors press decentralised governance, at times ignoring local cultural and institutional contexts.