Introduction: This study explores the level of assertiveness in various samples of students from Primary and Secondary Education. With the data obtained, on the one hand, we analyzed the relation between assertiveness and academic performance and, on the other, we verified whether students who are excluded from the norm, either because of their intellectual giftedness or because they display disruptive behavior in the classroom, obtain different scores in assertiveness. The study analyzes the relation between the excess or deficit of assertive skills and academic performance and, in turn, seeks to verify whether more rebellious students, from an academic viewpoint, or students who are intellectually gifted score differentially from the rest of the students of their level of assertion. Method: A sample of Primary and Secondary Education students from Spanish public centers, whose academic performance was normally distributed, was used. Students from Primary Education, selected for their high intellectual giftedness and good academic performance, who participated in a cognitive-affective development program, and a group of students from Secondary Education, integrated in Compensatory Education units, who were highly conflictive and who achieved very poor academic results, were analyzed. Self-rated assertiveness questionnaires were administered for comparison with the mean school grades and the number of subjects flunked. Results: A statistically significant negative correlation was obtained between the variables assertiveness and academic performance in the diverse samples analyzed. The data support the hypotheses proposed in the study. Significant negative correlations were obtained between performance in Natural Sciences, Social Sciences, Mathematics, Technology and the variable assertiveness; the correlation was lower when Language was also taken into account, and nonexistent when correlated with English. Discussion: From this study, it is concluded that, although an assertive deficit can act like a variable that predisposes towards academic failure, as it hinders students' interactions with teachers and classmates, a deficient assertive or passive dependent pattern can also generate behaviors that lead to very satisfactory academic results. However, the excess of this social skill or students' erroneous self-perception can originate a form of academic failure. (Contains 3 tables and 2 figures.)