In recent years more and more schools have recognised the important role they can play to support the mental health and social emotional wellbeing of their pupils. However, school staff are generally unaware of the scale of need because they rely on ad-hoc identification and do not conduct universal screenings of pupil wellbeing. In 2017 we launched the Boxall Childhood Project to campaign for schools to assess the wellbeing of all their pupils using the Boxall Profile, a teacher-led assessment tool of social emotional and behavioural difficulties. As part of the two-year pilot project, 40 English schools were recruited and trained to assess children using the Boxall Profile. Schools completed their first data collection in summer 2017 and more than 6,000 pupils were assessed. Overall, we found that pupils experienced high levels of social emotional and behavioural difficulties, but that little support was available to address their needs. We also replicated previous findings showing that, compared to girls, boys were experiencing higher levels of social emotional and behavioural difficulties. The current study provides an estimate of the scale of social emotional needs experienced by the UK pupil population and highlights the need for schools to provide more support.