Background: In Down syndrome (DS), adaptive behaviour often shows a “surplus effect” (i.e., higher adaptive abilities than expected from cognitive skills). As inclusive schooling has become mandatory in Italy, we studied the impact of school inclusion on the surplus effect of adaptive behaviour in adult DS, considering potential confounding factors such as parental education. Methods: All consecutive DS individuals from three different sites were queried prospectively regarding type of schooling (inclusive and non-inclusive). Demographic data were documented; cognitive abilities and adaptive behaviour were assessed (Coloured Progressive Matrices and Vineland Adaptive Behaviour Scales). The aim was to establish the presence of a surplus effect in adaptive behaviour, primarily in the overall level and secondarily in the main domains and subdomains. A multivariable-adjusted logistic regression model was used for the association of schooling, and parental education. Results: The majority (65%) showed a surplus effect in adaptive behaviour and had attended inclusive schools (85%). Higher adaptive skills as well as early and longer functional treatment programmes were more readily available for younger individuals. In the group of inclusive schooling, the surplus effect on overall adaptive behaviour was present in 70% as opposed to 38% in the group without inclusive schooling, significant when adjusted for gender and maternal education. This was also observed in socialisation, written, and community, and after adjustment in playing and leisure time. Conclusions: Adaptive behaviour showed a surplus effect in the majority of DS adults, even more so after inclusive schooling. Younger adults showed higher adaptive skills. Moreover, female gender and higher maternal educational level significantly enhanced this surplus effect.