It seems like that region had a particularly high literacy rate. Texts from all classes and genders were found. The kid could have been a peasant.

https://en.wikipedia.org/wiki/Novgorod_Republic#Literature_a...

How likely is that vs works written by the rich folk survive? A cursory GPT search implies not a lot of medieval works written by poor people in general

The point is that it's not your average medieval polity. Novgorod was a merchant republic in which literacy was seemingly held in high esteem in and of itself, resulting in a social pressure to acquire it. And then on top of that they had access to plenty of birch bark, which happens to be a very convenient and cheap writing material where it's available, and does not require any ink or fancy tools to use; just a sharp stylus.

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.

It would be cool to study if the social pressure was applicable to servants or just the elite ruling class

Do you have any idea where to get photos of collections?

If you can deal with Russian, this is listed as a "full database" on ru-wiki:

http://gramoty.ru/birchbark/

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.