Background: This randomised controlled trial took place in primary schools where teachers were already trained in the Incredible Years (IY) teacher classroom management programme and where the universal IY Classroom Dinosaur School social-emotional skills curriculum was being delivered as part of the statutory Welsh personal and social education curriculum. Aims: The study examined whether the IY Small Group Therapeutic Dinosaur School programme had added benefits for children with identified behavioural, social, and/or emotional difficulties. Method: Children were screened for behavioural difficulties using the teacher-rated Strengths and Difficulties Questionnaire and were eligible for study inclusion if teachers rated them as above the cause for concern cut-off on this measure. Two hundred and twenty-one children were randomised to intervention or wait-list control conditions. Assessments of behaviour and social-emotional competence were completed by multiple respondents, including teachers, children and researchers who completed blinded direct observations (on a subsample). Results: Multilevel modeling analyses showed improvements in the problem-solving knowledge of children in the intervention condition (ES = .39 for prosocial and .41 for agonistic solutions), compared to children in the control condition on the Wally Problem Solving measure. Intervention children were also significantly more likely to achieve teacher set social-emotional academic goals. Conclusion: This study shows limited but promising results for the addition of a targeted school-based social-emotional intervention delivered by teachers, alongside a similar class-wide universal curriculum, to young high-risk children.