I feel there are several reasons that people may not achieve the highest grades. I agree that self fulfilling prophecy plays a part-for many people, if they see no evidence that they are capable or perhaps it has been implied by teachers etc that they won't do well, then they lack the self belief to even try. After all, if you work hard and still fail its a crushing experience and won't spur everyone on to try even harder next time. I've also seen a friend do terribly in her GCSEs, and that was certainly largely due to her dyslexia.
When I did my GCSEs, I never understood how anyone could fail them as I personally found them pretty easy and came out at the end with great results from very little additional effort outside of school. However, I haven't had the same experience in my A-Levels which I'm just about to finish. I've suffered from serious clinical depression from just before I began my A-Levels, which definitely had a huge impact on my AS results. I believed I wouldn't even be alive by the end of Sixth Form, so I just couldn't see the point in anything including getting good grades.
I'm not trying to make excuses but it sure affected the amount of work I could do. I'm only just beginning to feel like myself again, now I just hope that I've managed to make up for my poor mental health in the past few years and get the grades I need. Although I think my grades have suffered due to my health, I also think that for people who are capable of simply cruising through GCSEs, A-Levels are not as easy to do this with. I know several people who did badly because the transition from GCSE to A-Level was a bit of a shock, and they'd gotten lazy and forgot how to work hard at studying.
So there are different reasons for everyone who does badly...