There is a growing interest in the factors influencing bystanders' interventions in school-bullying. However, perceived social norms, beliefs about what others do (perceived descriptive norms), and what others think one should do (perceived injunctive norms) still require attention. The aim of this study was to investigate whether students perceived three defending behaviours (alert, care and opposition) and one passive reaction as falling within social norms. Results from 5445 middle- and high-school students showed that alert, care and opposition defending behaviours were viewed as falling within perceived social norms, contrary to passive bystanding. Alert and opposition were perceived mainly as injunctive norms, whereas care was viewed as a more general norm. Multiple regressions showed that perceived injunctive norms and, mostly, perceived descriptive norms were both strong predictors of the readiness to adopt the four witness behaviours targeted in the study. The implications of these results for informing future debate and practical actions in schools are discussed.