Anxiety, depression and eating disorders show peak emergence during adolescence and share common risk factors. School-based prevention programs provide a unique opportunity to access a broad spectrum of the population during a key developmental window, but to date, no program targets all three conditions concurrently. Mindfulness has shown promising early results across each of these psychopathologies in a small number of controlled trials in schools, and therefore this study investigated its use in a randomised controlled design targeting anxiety, depression and eating disorder risk factors together for the first time. Students (M age 13.63; SD = .43) from a broad band of socioeconomic demographics received the eight lesson, once weekly.b (“Dot be”) mindfulness in schools curriculum (N = 132) or normal lessons (N = 176). Anxiety, depression, weight/shape concerns and wellbeing were the primary outcome factors. Although acceptability measures were high, no significant improvements were found on any outcome at post-intervention or 3-month follow-up. Adjusted mean differences between groups at post-intervention were .03 (95% CI: −.06 to −.11) for depression, .01 (−.07 to −.09) for anxiety, .02 (−.05 to −.08) for weight/shape concerns, and .06 (−.08 to −.21) for wellbeing. Anxiety was higher in the mindfulness than the control group at follow-up for males, and those of both genders with low baseline levels of weight/shape concerns or depression. Factors that may be important to address for effective dissemination of mindfulness-based interventions in schools are discussed. Further research is required to identify active ingredients and optimal dose in mindfulness-based interventions in school settings.