Adaptive behaviour is crucial for well-being and independence. The Adaptive Behaviour Assessment System (ABAS-3) is often used to plan individualised support in specialist educational settings, drawing on both teacher and parent informants. In practice, however, little is known about the extent of variation between informants. We examined informant discrepancies in a naturally occurring dataset of 28 ABAS-3 assessments (21 male, seven female), performed by teachers and by parents post-admission at an autism specialist school. We found that in real-life practice, teachers rated the adaptive functioning capabilities of their students significantly more highly than parents across all ABAS-3 domains, a discrepancy significantly greater than validation studies with this instrument, and interestingly, non-uniform across domains. The magnitude of this discrepancy was largest in the Conceptual domain, where differences between informants were greater in older children. The skill area most affected was 'self-direction', which includes aspects of executive function. Existing guidance for adaptive functioning assessment in specialist education is extremely limited; while considering potential sources of informant discrepancy, our findings corroborate the need for further guidance on the use and scope of tools like the ABAS-3 in educational settings. While such discrepancies may confound population-level data, individually, these skills may be indicative of greater need to support generalisation across contexts, and may indicate the particular skill areas that are most amenable to intervention.