According to this, a German study has found that consumption of violent media – including violent videogames – has a direct impact on a child’s school grades.
The German grading system runs from one to six, one being the highest and six the lowest. The study found that boys that played games rated for 16 year-olds and older were 0.1 to 0.5 below the national average, while those that played games rated for 18 year-olds and over were 0.3 to 0.66 below the national average. Those that didn't play games were, on average, slightly higher than the national median.
Violent games, therefore, make you stupid and train you to kill people. Really, who would have though that?