The study presents results of research conducted by means of the BPAQ and Cyber-Bullying Tolerance questionnaires on a sample of 788 elementary school pupils of the 8th and 9th grades. It shows an increase in verbal aggression, hostility and anger in the pupils. In terms of gender, the increase was confirmed in verbal aggression in boys and physical aggression in girls. Tolerance to cyber-bullying as a form of aggressive behaviour increases with the development of science and technology and the increase in pupils´ aggressive behaviour. Knowledge of manifestations and causes of pupils´ aggressive behaviour is the basis for the educational work of teachers and other professionals.