This study evaluated the effectiveness of a schools-based psychoeducational intervention designed to help teachers recognize the symptoms of clinical depression in their adolescent pupils. Around 151 teachers in eight high schools in Scotland, UK were randomly assigned to experimental and control groups and all received training on depression. The ability of the experimental teachers to report which pupils were depressed was compared with the control group whose reporting task occurred before they had received training. The teachers were reporting on 2262 pupils who had been independently screened for clinical depression using a two-stage screening procedure with the Mood and Feelings Questionnaire (MFQ) and semi-structured clinical interview (K-SADS). Systematic evaluation showed that training teachers with this package did not improve their ability to recognize their depressed pupils. Recognizing depressive illness in adolescence is one of the main public health challenges for adolescent mental health services and this study adds to the growing literature on the difficulties in achieving this.