This study was a controlled evaluation of a comprehensive school-based universal prevention program involving male and female students, parents, teachers, school administrators and local public health professionals. A total of 982 male and female Grades 6 and 7 middle school students (and 91 teachers/school administrators) completed self-report surveys at baseline on measures of body satisfaction, internalization of media ideals, size acceptance, disordered eating, weight-based teasing, weight loss and muscle-gaining behaviours, and perceptions of school climate (teachers only). Eighty-four percent of the students repeated the surveys immediately following the 8-month school-wide intervention and 71% again 6 months later. Repeated measures ANCOVAs revealed that participation in the Healthy Schools-Healthy Kids (HS-HK) program had a positive influence by reducing the internalization of media ideals among male and female students and by reducing disordered eating among female students. The program was also associated with reductions in weight-loss behaviours among the students, although this effect was lost by the 6-month follow-up. When the intervention students were sub-divided into low versus high-risk groups, the high-risk group appeared to benefit most from the intervention with significant reductions in internalization of media ideals, greater body satisfaction, and reduced disordered eating over time. There were no intervention effects for teachers. Challenges of engaging teachers in prevention are discussed.