There is increasing disconnect between children and nature in the UK. Given that childhood nature experiences are important for the development of nature connection, well-being benefits and aspects of children’s development, mapping the existing biodiversity that children are exposed to on a daily basis in schools and their grounds is vital to identifying areas of low biodiversity and to developing strategies to increase exposure to nature. Despite children spending a large portion of time at schools, there has not yet been an in-depth, in situ assessment of the biodiversity present in school grounds. Using a sample of 14 English schools, including state-funded and non-state-funded schools, we used remote images to quantify green-space area within a 3–km buffer around (buffer greenness) and within (school greenness) each school, including the school’s grounds surrounding the buildings, and in situ images to quantify vegetation visible to children within each school’s grounds (visible vegetation). We also surveyed trees, ground plants, ground invertebrates and birds within school grounds. School greenness correlated positively with visible vegetation, but buffer greenness was not related to either school greenness or visible vegetation. Buffer greenness correlated positively with plant richness, and school greenness correlated positively with tree abundance and richness. Visible vegetation correlated positively with tree abundance and richness, maximum tree diameter at breast height, plant richness and invertebrate abundance. Non-state-funded schools had higher visible vegetation than state-funded schools. Our sample indicates that schools can support considerable biodiversity and that this is broadly consistent across state-funded and non-state-funded schools. We suggest that increasing the amount of vegetation, through planting of trees, shrubs and borders, may be the most effective method of increasing school biodiversity, as visible vegetation had effects on the greatest number of taxa.