The Boxall Profile is one of the most frequently used tools to measure the social, emotional, behavioural and/or mental health difficulties (SEMH) of children and young people in schools across the UK. However, limited evidence exists regarding the norms and psychometric properties of the tool. The current study used data from 487 primary school-aged children to carry out a re-standardisation and explore a range of psychometric properties of the Boxall Profile. Updated norms were established for 4- to 11-year-old children using a Receiver Operating Characteristic approach and the Strengths and Difficulties Questionnaire as gold standard measure for SEMH difficulties. Psychometric analyses indicated good internal consistency, concurrent validity and convergent validity but also highlighted the need for further research into the structural validity of the tool. To our knowledge, this is the first study providing substantial evidence for the reliability and validity of the Boxall Profile since its publication in 1984.