The aim of this paper is to analyse the behaviour of school violence bystanders from a game theoretical perspective. First, I describe the problem of bullying concentrating on the role of bystanders. Then I focus my attention on the witnesses of violence who either ignore a victim or try to defend him/her. A bystanders' decision consists of different elements and includes different stages, such as interpretation of the event and consideration of possible costs of intervention. In the article two models concerning two different stages of the decision making process are used to analyse the behaviour of bystanders: model of informational influence and a critical mass model. Informational influence model relates to interpretation of an act of violence as demanding or not demanding intervention. The observers decide whether intervening is an appropriate reaction, taking into account the behaviour of their classmates. This model shows the phenomenon of pluralistic ignorance and some factors that can influence its appearance. In the critical mass model, the bystanders assess the chances of success of their reaction in context of perception of the bully's power. This model is based on a public goods game model and it shows factors influencing the possibility of bystanders intervention.