I'm in my second semester of college. I smoked multiple times just about every day and I got a 3.6 GPA. Since I'm not sure if they use that system in Europe, that translates into an A-, which is hardly a grade you'd expect a "stoner" to get. It's all propaganda, how they try to tell you that weed can affect your schoolwork. This is how it works- If you are motivated enough to do well in school, you will. Just manage your smoking responsibly and don't let it seriously cut into study time that you put aside for yourself. There's no secret to it; you just need to use your head.