This study aims to analyse the relationship between moral disengagement mechanisms, aggression and bullying behaviour in 221 high school students between 11 and 17 years who took the Mechanisms of Moral Disengagement Questionnaire, The Aggression Questionnaire and the School Bullying Questionnaire. Findings suggest significant positive correlations (p < 0.001) between aggressive, bullying behaviour and moral disengagement. Structural equation modelling shows that moral disengagement explains 14% the variance in verbal aggression and 34% the variance in physical aggression. Moral disengagement and verbal aggression explain 10% the variance of negative symptoms related to anxiety, depression, post-traumatic stress and effects on self-esteem. Finally, moral disengagement, negative symptoms, physical and verbal aggression explain 65% the variance of bullying behaviour. These findings can support the development of new and effective educational interventions.