In this paper, we present the results of an experimental sequence of classroom activities on geometric transformations, proposed to high-school students. The activity is based on the use of a dynamic geometry system. Planned by a group of four people, two university professors and two research teachers, the activity has been elaborated together with 16 teachers who experimented in their own classrooms. The results of the classroom activities have been obtained by analysing teachers' log books and questionnaires submitted to the teachers and the students.