The aim of the present study was to conduct an analysis of TIMSS (Trends in International Mathematics and Science Study) 2003 database and to determine how negative school factors, such as aggression, are associated to the mathematical and science achievement of students. The analyses were conducted separately for national and international data. National analyses for Slovenia show significant associations between math and science achievement and the experience of aggressive behaviour. Students who experienced aggressive behaviour scored lower in math and science, both in the fourth and in the eighth grade. The results of the regression analysis show that negative factors, such as aggressive behaviour, are good predictors of educational achievement in Slovenia. International analyses for the selected countries (high- and low-achieving countries from the whole TIMSS population) confirm that this type of finding is culturally impartial as well as valid for the level of achievement both in math and in science. (Contains 8 tables.)