> ...secondary school pupils aren't able to pass maths and physics exams from 30 or 40 years ago.

On its own, that isn't a particularly useful observation, because more than just the test has changed since that time. For instance, teachers who seek to help their pupils pass a test teach, to a greater or lesser extent, 'to the test'. Are the present-day students being taught to a test from four decades ago? This is just one of many factors which one would need to control for in order to accurately compare performance over time. Although there are certainly people who specialise in that research, I think it is more useful to ask what skills our present-day society needs, and work back from there. There are vanishingly few professions in which a knowledge of the number of primes, say, has any relevance. What do people need to know now, and what books should be read by students in order to learn it?