The rise in prevalence rates of children with ASD is well documented internationally. In the United Kingdom, Northern Ireland has consistently higher rates although the reasons for this are not clear. Using data from the annual school census that is held each year in Northern Ireland, changes in the numbers and prevalence rates of pupils identified by schools as having ASD, were examined over a nine-year period from 2010/11 to 2018/19. Pupils living in areas with greater social deprivation had higher rates of ASD. Similarly pupils attending non-selective secondary schools had higher rates than their peers in grammar schools. In recent years there was an increase in pupils assessed by schools rather than in the formal statementing process. Also more girls were identified as having ASD as were pupils aged eight years and above. The factors associated with increased prevalence rates in Northern Ireland are confirmed by the wider literature. The study illustrates how improved practice and greater equity in supports to schools and pupils with ASD could result from comparative analyses of a total population census that is regularly updated.