The (state-run) education system in England and Wales (and notably not Scotland), as it was set up in the 1950s:

At the age of ten, children were required to take a set of examinations, popularly known as the eleven plus as the results affected your secondary education (that after the age of 11).

Those who performed well in the exams went off to Grammar Schools where they were taught traditionally academic subjects and basically primed for University and a future professional career.

Those who proved less able at the tender age of ten, were shunted off into Secondary Modern Schools where they were taught basic skills and groomed to be society's future clerks and factory drones. Getting into university from a Secondary Modern was highly unlikely, however polytechnics provided degree-equivalent education for the more able of the majority of the population.

In the 1970s and 1980s fewer and fewer local education authorities continued using this method of separating children, as it was generally believed that an exam at 10 years old should not be allowed to influence someone's entire life. Many phased out use of the 11 plus and the Grammar/Secondary Modern School distinction, putting pupils of all abilities together in Comprehensive Schools. In the late 1980s the two-tier examination system was replaced by a single unified qualification at 16, the GCSE and a couple of years later the polytechnics were allowed to become full universities. Grammar schools can still be found in a few parts of the UK however, notably Northern Ireland.

Of course this only applies to the vast majority of the population who go through publically funded state schools; those wealthy enough to afford private education were never affected by any of this.