This is how it used to work. At age 11 people had to take an 11-plus exam, a test of cognitive ability, of which roughly 20% passed to go to Grammar School (now abolished although you can still go privately), who were thought capable of an academic education, meaning having the ability to pass the standard five O-levels that employers wanted for entry level jobs. It was also the stepping stone to taking three A-Levels to get into a degree course.
Once at grammar school you were streamed after the first year, so the top stream did all three sciences as separate subjects, two languages, Maths, English Language, English Literature, History and Geography from year two to take nine to ten O-Levels in Form 5.. The second stream did two sciences and one language plus art and music, the two lower streams did six or seven subjects plus Art and Domestic Science for girls and Woodwork for boys. The syllabus was demanding and we were expected to do at least two hours of homework every night and swot like mad in very competitive exams for which you were ranked. Coming top was the goal. I was certainly a diligent girlyswot coming top or near top in most subjects.
The grammar schools were abolished as they were considered unfair to the less able pupil - the other 80% - but on the other hand they enabled social mobility, so someone from a poor home could have the same first class education as a rich kid, simply because of innate ability.