I would suggest that the % of very bright, bright, reasonably bright, not so bright and those for whom most things will be a struggle remains fairly constant from one generation to another. The problem is that the demands made of each generation from an educational point of view are being diluted. Teachers can, with the best will in the world, teach only what the syllabus in their particular subject decrees and "teaching to the test" in order to maintain the school's place in the league table is the norm.
I used past "O" Level German comprehension papers to help prepare "A" Level candidates for that section of their exam. I marked "O" Level until ii was abolished and have marked G.C.S.E since 1988. Candidates can only do what they are asked to do --- and many of them, given the opportunity, could and would do well even with a more demanding challenge.
As for the headteacher who did not know where the Mendips are, all I can say is that I was always surprised what my headmaster did not know when I presented the annual staff quiz. He was, however, a fine mathematician and enthused the few classes he taught with his love for the subject.