Keep Cool … Start at School (KCSaS) is an evidence-based cognitive behavioural prevention programme for children aged 8 to 12 years with externalising problems in a difficult-to-reach population. Externalising problems are often accompanied by additional challenges in childhood and can continue into adulthood, negatively impacting the individual. The prevention programme’s effectiveness was initially tested in a research study by Liber et al. in 2013. In the past years, the programme has been implemented in community care. However, it has not been investigated yet if today’s prevention programme in community care is as effective as it was in the original research study conducted in 2013. This study aimed to compare the behavioural outcomes of children who participated in the KCSaS intervention in community care versus the research study setting. Community care participants (n = 23) were matched to research study participants (n = 23) on gender and the level of pre-intervention problem behaviour. To test the intervention’s effectiveness, pre- and post-intervention scores on the Strengths and Difficulties Questionnaire (SDQ) were compared within and between groups using dependent t-tests and a repeated measures ANOVA, for both children who participated in a community care and research study setting.