From Gamer.tm: "A study conducted by the Kriminologischen Forschungsinstituts Niedersachsen (KFN) in Germany has found that the consumption of violent media – including violent videogames – has a direct impact on a child's school grades.
"Using boys as a sample group, the study found that the average grades for German, maths and general studies for children who regularly play games cleared for ages 16 and above were 0.1 to 0.5 below the national average. By contrast, boys who never play such games typically receive grades slightly above this.
"In the case of games cleared for ages 18 and above, the effect was even more profound. According to the results, the average grades of children who regularly played such titles were 0.3 to 0.66 below the national average, while those that never played such titles were again marginally above.
German schools use a numerical grading system, where one is the highest grade attainable and ten the lowest. All age ratings were based on those awarded by the Unterhaltungssoftware Selbstkontolle (USK) board."