Background: Although studies have long identified the practices that must be included in transition programmes to improve the outcomes for young people with intellectual disabilities when becoming adults, less information is available regarding the degree to which those best practices are being developed and/or implemented. Materials and methods: This exploratory research assesses the implementation level of those best practices in a school programme to facilitate the transition to adult life in the Basque Country, Spain. The opinions of professionals were asked through an ad hoc questionnaire, and a case study of three schools was conducted through observation, documentation analysis, interviews and focus groups. Results and discussion: The professionals were found to have a positive opinion of the implementation degree of the best practices, except for the inclusion dimension. Although analysing the cases partially confirmed this opinion, contradictions emerge as these best practices cannot always be identified in the running of schools. The dimensions with major discrepancy are related to the individualised transition planning, family involvement and interagency collaboration. Conclusion: Despite having identified best practices in all dimensions, delving deep into the professionals' understanding concerning best practices is crucial.