In the context of inclusive schools, social-emotional learning encourages student involvement in classroom life and is related to a decrease in maladaptive behaviour. The objective of this study is to analyse the impact of a social-emotional education program on aggressiveness and emotional instability in childhood. Participants were 555 children aged 7 to 12 years (M=9.2 and SD=1.5), 55.5% boys and 45.5% girls. The children were students of primary education at six public schools in Valencia (Spain). The sample was distributed into an experimental group (317 children; 57.2% of the total participating population) and a control group (238 children). The teachers of the experimental group received training to implement the program in class. The teachers of the control group received no training and did not apply the program. The results in the pre-test and posttest phases in both groups are analysed. In the pretest phase, significant differences appeared between the groups: the experimental group showed higher levels of aggressiveness and emotional instability than the control group. In the post-test phase, aggressiveness and emotional instability decreased significantly in the experimental group (medium-high effect size), whilst aggressiveness and emotional instability increased in the control group. The effects of the program on the students are discussed further.