Check In–Check Out (CICO) is a support programme for students with externalising problem behaviours. This study implemented in Finland is one of the first reports on the application of CICO in the European context. An experimental, multiple-baseline, single-case design is used to examine the effects of the CICO intervention on the problem behaviour and appropriate behaviour of three general education primary school pupils. The similarity of the observation data and the information provided in daily report card (DRC) are investigated to gauge the validity of the DRC assessments. The fidelity and social validity of the intervention are also analysed. CICO reduced problem behaviour in all pupils. The results suggest that DRCs are valid tools for estimating behaviour in applied settings. CICO was implemented with high fidelity, and it had excellent acceptability among school personnel. This study adds to the existing literature by implying that effective behaviour support for pupils with disruptive behaviours can be applied easily in general education settings and in diverse international contexts.