In this paper, we deal with a classroom where bullying is occurring and assume that if a number of bystanders who tattle bullying exceeds a threshold, then bullying is stopped. Bystanders’ behaviour is formulated as a non-cooperative game, with negative externality, retaliation cost and reporting cost. Our research shows that to stop bullying, it is useful to raise the cost of negative externality and to reduce the retaliation cost and or the reporting cost. Additionally, it is shown that small class is effective in preventing bullying.