An annual school census is held in the four countries of the UK in which pupils with ASD are identified. This longitudinal study of the total UK school population compares increases in prevalence rates for pupils with ASD over a nine-year period from 2010/11. Northern Ireland had the highest prevalence throughout, reaching 3.20% by 2018/19 compared to 1.92% in Wales. The prevalence rates were higher in secondary than in primary schools. The increases in prevalence and intra-country variations may be linked to a greater appreciation of ASD occurring alongside other developmental difficulties. Greater reliance on school rather than statutory assessments may also contribute to increases in pupils with ASD. More in-depth research is needed to establish the reason for the variations across years and countries.