Around 2000, we started to propose to students exercises with mechanized grading. Since, we have been accumulating lots of data that confirm the evidence: the more the students practice, the better they perform at examinations! Therefore, to foster the use of these exercises, we devise a ranking system where every student may compare his skill to the others. As for video games, we expect students to be attracted by this feedback. Computing these skills also have some interesting side-effects mainly on the suggestion of interesting new exercises.