Twelve-month follow-up outcomes from a group-randomized trial (GRT) of a classroom curriculum aimed at preventing bullying and victimization among elementary students in the Denver, Colorado, public school system are presented. Twenty-eight elementary schools were randomly assigned to receive selected modules of "Youth Matters" (YM), a skills-training curriculum that targets bullying and victimization, or to a no-treatment control group. Linear growth models were fitted to five waves of data collected over 3 years to test the effect of the intervention on the rate of change in self-reported bullying and victimization. Participation in YM was associated with a 7% decline in bully victimization 1 year after the intervention ended. Practice and methodological challenges encountered in the investigation are discussed in the larger context of intervention research in school settings. Strategies to increase school-based intervention research by social work investigators are outlined. (Contains 2 tables and 1 figure.)