Sharing a classroom environment with other victimized peers has been shown to mitigate the adverse effects of peer victimization on children’s social and psychological adjustment. By extension, this study hypothesized that classroom reductions in the proportion of victims would be harmful for children who remain victimized. Data were collected at the end of 2 subsequent school years from 4,466 fourth- to sixth-graders (mean age = 11 years), as part of the implementation of the Finnish anti-bullying program KiVa (an acronym for Kiusaamista Vastaan, “against bullying”). Multiple regression analyses were conducted on a subsample of 170 stable victims (children reporting being victimized at least 2–3 times a month at both time points) to test whether a decrease in the proportion of victims in their classrooms had an effect on their adjustment at Time 2. Stable victims felt more depressed, more socially anxious and were less liked at Time 2 in classrooms where the proportion of victims had decreased in 1 year compared to stable victims in classrooms where it had increased or remained the same. These effects were not moderated by the intervention status of the classroom. Paradoxically, an improved social environment can be detrimental for some children. These findings point to the necessity to maintain anti-bullying intervention efforts especially when successful.