This study, carried out with 10,795 Compulsory Secondary Education students (Asturias, Spain), seeks to determine what behaviours are considered school bullying, to what extent they are witnessed, and what the typical reactions to bullying are. To gather the data, a 19-item questionnaire was used, with a high degree of reliability (α = 0.85). Regarding the results, most of the students have a concept of bullying generally related to the use of physical force, although the behaviours witnessed with the greatest frequency are insults, social exclusion, or damaging belongings. As for their reaction, students tend to assume the role of a proactive witness, the tendency being to ask an adult for help. Nevertheless, there are many students whose knowledge and recognition of bullying is scarce, and who, by choice, limit themselves to observing. The results are useful for designing educational actions aimed at the intervention and prevention of school bullying.