There's a various amount of reasons as to why various people do not acquire a pass. This could be because of home life, many people I know lost someone tragically close to them right before GCSE's and so you can understand why they wouldn't come out of it with the best grade they had the potential of achieving, but even how you raise your kids will be a defining factor as they'll follow in the parents footsteps, many people local to me are 'druggies' and care nout for child's educational future.
The child's personality could also differ a D grade to a C grade, as they haven't matured, once again this could be down to their parents or perhaps even social media where they see other youths messing about? They do not see reason when it comes to education and therefore do not try. They could even just be bone idle - I've eavesdropped on my conversations where, yes, they plan to go on benefit once they're done with college, and yes, 'like their parents do'.
Teachers could also play a role with how they construct their lessons. Apart from the 'I don't like my teacher' or 'I hate this subject', it could be just too boring for the, that they consistently loose focus? Maybe they could be a strong hands on learner as the teacher usually only plays to the traditional sense of learning and people who learn differently cannot blossom.
There could also be a large percentage of disabled people who, to whatever illness, cannot learn and take in information as others can.