My favorite example is the writings of Onfim, who was a little boy in the 1200s in present day Russia whose scribbling and homework were exquisitely preserved on birch bark fragments. It’s so immediately recognizable as a little boy’s endearing doodles about knights and imaginary beasts, yet its 800 years old.
insert David Lynch quote about red ants
https://en.wikipedia.org/wiki/Novgorod_Republic#Literature_a...
As far as survival, because of how common birch bark notes were in Novgorod, they were used for mundane everyday matters like instructions to servants to purchase such and such goods, or even just a one-liner greeting, and a lot of those were then discarded and ended up buried underground. So, again, a very different slice compared to the more typical story of expensive manuscripts in libraries, or personal correspondence of nobility.
I don't know if something similar exists in English.
There are plenty of notes written by peasants and servants, so it clearly wasn't just an elite thing. But IIRC the vast majority of those letters are from the city proper, and Novgorod Republic was a city-state governing a large territory outside of its walls, so we're talking mostly about the urban population.
Keep in mind also that this is an actual republic with all male citizens voting in a public assembly on important matters, so it would make sense for the culture to appreciate the trivium more universally.