We report on a long-term study which was executed in a German secondary school with 128 eights graders (ages 14 to 15) in four different classes. Two of these classes served as control groups. The mathematics lessons of the other two classes (treatment groups) were frequently enriched by distinguished phases in which structured argumentation and the use of heuristics was trained. The study aimed at investigating the development of the argumentation competence of the students over that period. For this report, the products of four different geometry tasks of 15 students from one of the treatment groups and 15 from one of the control groups respectively were evaluated. [For the complete proceedings, see ED597799.]