Introduction: School bullying is a public health concern due to its impact on the present and future of schoolchildren, where the role of bystander becomes more relevant every day. Objective: To describe the knowledge about the systems for reporting bullying and the type of intervention of adolescent bystanders in public schools in Lima. Methods: Cross-sectional and descriptive study carried out with 668 students chosen from public schools in Lima, Peru, by nonprobabilistic convenience sampling between August and December 2019. A survey and a questionnaire permitted to assess the type of intervention in the face of bullying and knowledge about systems for reporting bullying. Pearson’s chi-square test was used for data analysis with a significance level of P<0.05. Results: A situation of school violence among peers was witnessed by 59.82 % of the respondents, 63.34% of which intervened in this situation. Based on sex and year of study, there were statistically significant differences in the percentage of schoolchildren who intervened. 51.39 % defended the victim or talked with the aggressor. Regarding knowledge about school violence reporting systems, 24.21 % know about the existence of SiSeve, while 64.73 % knew DEMUNA. Conclusions: The main type of intervention of students who witnessed a bullying situation was to defend the victim or talk with the aggressor. On the other hand, there is little knowledge about the existence of SiseVe among the students, although most of them knew about DEMUNA.