A cluster randomised controlled trial to determine the clinical effectiveness and cost-effectiveness of classroom-based cognitive–behavioural therapy (CBT) in reducing symptoms of depression in high-risk adolescents P Stallard, 1 * R Phillips, 1 AA Montgomery, 2 M Spears, 2 R Anderson, 3 J Taylor, 4 R Araya, 2 G Lewis, 2 OC Ukoumunne, 5 A Millings, 1 L Georgiou, 1 E Cook 1 and K Sayal 4 1 Department for Health, University of Bath, Bath, UK 2 School of Social and Community Medicine, University of Bristol, Bristol, UK 3 Peninsula Medical School, University of Exeter, Exeter, UK 4 School of Community Health Sciences, Institute of Mental Health, University of Nottingham, Nottingham, UK 5 PenCLAHRC, Peninsula College of Medicine and Dentistry, University of Exeter, Exeter, UK * Corresponding author Declared competing interests of authors: Paul Stallard holds other grants paid to his institution from the National Institute of Health Research (NIHR) and has received funding to speak at the Excellence in Paediatrics Conference in Istanbul in December 2012. Glyn Lewis has grants/grants pending to his institution from the NIHR, Medical Research Council, Wellcome Trust, and US National Institute on Alcohol Abuse and Alcoholism that are relevant to this work and has received funding to speak at the Austrian Society of General Practice and Family Medicine conference, Vienna, in 2012. Abigail Millings is currently employed by Ultrasis UK Ltd and has stock options with her employer. All other authors declare (1) no financial support for the submitted work from anyone other than their employer; (2) no financial relationships with commercial entities that might have an interest in the submitted work; (3) no spouses, partners or children with relationships with commercial entities that might have an interest in the submitted work; and (4) no non-financial interests that may be relevant to the submitted work. Published October 2013 DOI: 10.3310/hta17470