The study of violence and bullying in schools is a line of scientific research that has contributed significantly to knowledge on human aggressiveness, especially in children and adolescents. This article shows that there are two patterns of aggressive behavior: proactive and reactive. Both are present in bullying, as are other psychological aspects pertaining to the individuals involved, such as basic personality traits, self-esteem and values. This study links both proactive and reactive behavioral patterns to involvement and non-involvement in school bullying. The results reveal that basic personality traits, such as neuroticism, have a direct impact on proactive and reactive bullying, as do the social and individual dimensions of self-esteem and social and moral values. These findings confirm that variables relating to personal and social values are, in turn, related to proactive and reactive aggressive behavior in bullying for those involved and not involved in bullying. However, they also highlight that while aggressors engage in more proactive aggressive behavior, reactive aggression is more frequent among victims.