Sejong the Great famously invented Hangul, the writing system so easy that it basic literacy can be reached in only a few weeks. So clearly after hundreds of years the entirety of Korea should be pretty dang literate, right? Yet ingame they have one of the lowest among the major nations.
Turns out Hangul wasn't adopted until 1894 with the Kabo reforms, until then they were still using the infamously hard to learn Hanja system. Literacy rapidly increased after that until Japan conquered Korea and imposed the Japanese language on them, shattering their literacy again.
To my knowledge there's no Event or Journal Entry for the revival of Hangul. I would suggest upon researching Nationalism allowing you to convert the official writing system to Hangul giving a massive boost to literacy rate growth, but at the cost of severely ticking off the Intelligentsia Landowners, and Confucian Scholars (unless somehow Korea has become Buddhist in which case it would historically please the Buddhist Monks IG as they were the ones who kept Hangul from dying)