Children spend more time in school than in any other formal setting and, with mental illness in children on the rise, there is more pressure on schools to intervene in student mental health than ever before. In the current study, two phases of semistructured interviews were conducted with school leaders and special educational needs coordinators (Phase 1, N = 23; Phase 2, N = 11), to investigate first-hand experiences in dealing with student mental illness. Thematic analysis, drawing on Grounded Theory, was used to identify themes. The results identified deprivation as one of the main causes of mental ill-health in students, with insufficient budgets, inappropriate mental health services, and overly long waiting times as barriers to intervention. Difficulties in identifying appropriate mental health interventions to use in school were also reported. The authors propose a simple four-point heuristic, for assessing the quality of school-based mental health interventions to be used by school staff, so that educators can more readily identify appropriate mental health support for their students.