The wellbeing of young people has become an important education policy issue, with suggestions that mental health problems amongst young people have increased in recent years. Experiences at school are thought to be a key factor contributing to mental ill-health amongst adolescents. Yet surprisingly, little is known about how mental health outcomes vary across school year groups, independent of the effects of age. This article contributes new evidence on this issue, drawing upon large-scale health data from England. We find substantial growth in mental health problems as young people progress through secondary school. Yet this seems to be driven by the effects of age, rather than due to movement into more senior school year groups. We consequently conclude that evidence of a direct link between school year group and young people's mental health remains relatively weak.