In the scientific literature, it is emphasised that it is essential to address challenging negative behaviours in the classroom in order to maintain a conducive learning environment. This article aimed to describe oppositional negative behaviours in Salvadoran secondary school students, in order to make suggestions for prevention and intervention in school violence. The type of research was mixed, with a concurrent triangulation design (DITRIAC) and an exploratory-descriptive scope. In the quantitative phase, a survey was applied to 148 students (52.7% female and 47.3% male). In the qualitative phase, an in-depth interview was conducted with four principals and two psychologists. The results indicate that the students presented disrespectful behaviours towards teachers and classmates, as well as resentful and vengeful attitudes. Both the principals and the psychologists point out that these behaviours extend to the use of social networks, where students publish offensive and threatening content regarding their peers and authority figures. The practical implications of the findings are discussed, and it is concluded that it is essential to promote psychoeducational interventions with an emphasis on the development of emotional regulation strategies that enable the reduction of oppositional behaviours and the prevention of school violence.