Students with mental health difficulties are at increased risk for victimization, and this risk may be exacerbated during the transition to middle school, when there is an increase in bullying behaviors. Through a social-ecological lens, the present study investigated how internalizing and externalizing problems in the fall of fifth grade were associated with bullying role behaviors in the fall of sixth grade and whether these associations differed by gender. This study expanded prior research by examining not only bullying and victimization, but also bystander behaviors (i.e., assisting, defending, or outsider behavior). Participants were students attending fifth grade at one of 13 elementary schools at Time 1, and later attending sixth grade at one of five middle schools at Time 2 (N = 1,139). Structural equation modeling results indicated that externalizing problems in fifth grade were positively associated with bullying (B = 0.25, p < 0.001), assisting (B = 0.08, p = 0.024), and victimization (B = 0.30, p = 0.010) in sixth grade; further, internalizing problems in fifth grade were significantly positively associated with victimization (B = 0.43, p < 0.001) and defending (B = 0.33, p = 0.002) in sixth grade. There were no significant gender differences in these associations. Implications for social-emotional screening and intervention are discussed.