Actually I agree with BoffinMum, at least about languages GCSE which is what I know about. From what I've seen of my own children's GCSE work, I certainly believe that most children, given an environment that is free of petty low-level disruption, resource shortages and excessive staff absence (usually due to stress and depression rather than the fecklessness some would like to think it is), and with reasonably competent subject knowledge on the part of teachers, most children would achieve at least a B. There is nothing mysterious or mythical about a B. GCSEs are really not that hard if you've sat through 5 years of secondary schooling, progressing to more difficult work each year. A year 7 who listens and works hard could get a D in languages GCSE. So, in year 11, they should be doing a lot better than a D, you should think.
My experience of the "bog-standard" comprehensive however was that many pupils become more and more reluctant to learn anything new beyond age 12/13, with the result that many stagnate from round about year 8/9 onwards. This is irrespective of teaching quality- even very good teachers note this happening. Some children simply seem to make a decision to not get any better at the subject, despite the fact that they are perfectly able to.
There is almost a culture of mediocrity- it's seen as uncool to learn anything that your teachers want you to, pupils get bored and need to entertain themselves whilst not doing any work, so they turn to low-level disruption, thereby scuppering the chances of those who do want to learn of actually getting anything done, as the teacher then has to spend time dealing with disruption. Pupils who do well despite that environment are the ones where the lesson is written on the board, with exercise and page numbers, and are thus able to follow the lesson by themselves with little or no teacher input.
Appeals to parents to help with discipline issues often result in lame excuses: "It's a personality clash", "I was never any good at x subject so little johnny won't either", "Little Johnny is as good as gold at home, it must be something you're doing wrong", all of which amount to a refusal to support the teacher in their attempts to deal with disruption. All you need is 2-3 like this per class to really ruin a whole lesson. What you get in many schools is 8-9 per class. With a very firm hand, it is possible to make around 5-6 back down again, but you are still left with an intractable chorum of 2-3 disrupters. Your only hope as a teacher is to get the parents of one or two on board, leaving one out on a limb with no cronies to support him/her.
My friend gave me a copy of Machiavelli's "The Prince" when I qualified as a teacher. She'd been teaching for 8 years by then and had wisdom.