Compulsory education provided to all people is a communist staple, to the point right-wingers tend to point to this fact as to why the US shouldn't fund public education, since that's communism.
NK under went the single largest reduction in population of any country in world history during the war, thanks to the ridiculously violent and thorough bombing campaign from the invaders in the south. This massive reduction in population plus total state control of remote villages for security (due to constant incursions from the South necessitating security in all remote areas) means getting supplies to those areas becomes a priority, and there aren't many supplies needed to accomplish goals like the massive education and literacy programs of the 1970s and 80s that, for a time, accelerated past China's own accomplishments, and by percentage of population beat the USSR's record.
Or, if you want the neoliberal explanation, it is easier to brainwash those poor wittle villwagers if they can read the pwopaganda and not just listen to it.