From my own observation, I've noticed that high school psych students (usually the ones who only take the first-level courses, even if repeatedly) are often pot heads. Wanting to try other drugs and knowing the symptoms of depression, they simply go to their psychiatrist, portray the symptoms and get more drugs. With how much the term is thrown around these days, it is the easiest psychological ailment to fake.
I'm not saying that's always the case, but I saw so many people do it when I was in high school, that I'm sure it's skewing the results even if just slightly.