The theory says that education especially higher education was designed to show conspicuous consumption. To this end it was essential to study something of no monetary use i.e. I'm so rich I can study solely for effect and not to land a job.
Ideal subjects were dead languages, Latin, Greek, etc. Archeology, History, Geology, Geography, and so on. This is the reason that Pure Mathematics and Theoretical Physics were made separate from Applied Mathematics (and Physics) and could thus be included as Oxbridge degree subjects.
Of course, the prime minister excepted, not many of us can afford to study something that is not vocational and so we started to hear how Latin is actually useful because it teaches us how to name plants and describe medical complaints! Geography and History train the brain etc. As an aside, it is strange that while there are apologists for Latin there are none for Greek despite the extensive use of the Greek alphabet in Science and Engineering.
While at University I struggled with the Mathematics in my Engineering course and failed completely at partial differential calculus, I mean who would ever use this stuff? Subsequently I spent years acquiring and interpreting pressure transient data on gas wells in North Sumatra. I'm sure fate just wanted to push my face in things.
But back to school, which subject least useful? Definitely an O'level in Religious Studies (The Synoptic Gospels). I only had 7 subjects at O'level because I had dropped Latin. That meant I was studying Mathematics, Physics, Chemistry, English, French, Technical Drawing and Religious Studies.
The sixth form was a chance to study what I liked and what I found easy. Is there a connection? And my A'level grades spelled the name of a well known Swedish pop group.
In the middle 70's, when Universities were essentially free, nobody seemed to want to study engineering so I waltzed into Imperial College.
Yes I am a little full of myself and you are free, though not obliged, to remind me of the fact.
Cheers, Colin