Introduction: Social competence in school students has been studied extensively in terms of their being socially competent or not. However, there has been little analysis of how teachers contribute to the development of these skills. This research assesses the influence of teachers' assertiveness on the social competence of their students and on class-group cohesion. Method: The sample consisted of 36 teachers and 302 sixth-year Primary School students. Teachers filled in the ADCAs Self-assessment Report on Attitudes and Values in Social Interactions in its ADCA-pr form, and students completed the AECS Social Attitudes and Cognitive Strategies report, and form A of the BULL-S Inter-Pupil Aggressiveness Assessment Test. A first linear regression analysis took attitudes comprising social competence and social incompetence as dependent variables. Independent variables were self-assertiveness, hetero-assertiveness, the teacher's age and gender, and the student's age and gender, as modulators of the effect. We also used a Generalised Estimating Equation model in which the dependent variable was group cohesion and the independent variables were self-assertiveness, hetero-assertiveness, and age and gender of the teacher. Results: Results showed, on one the hand, that the teacher's hetero-assertiveness affected students' social competence, lessening their dominance (antisocial attitude), and on the other, that both self-assertiveness and hetero-assertiveness had an impact on group cohesion, although this effect was mediated by the gender of the teacher and by the construct itself. Discussion and Conclusion: It is confirmed that teacher assertiveness positively influences the development of students' social competence, and more specifically, that of students who manipulate and take advantage of others for their own benefit and feel superior to them, while increasing cohesion among members of the class. Therefore assertiveness appears to be a desirable interpersonal skill for professional teachers.