One of the first things North Korean defector Ri Kwang-myong did after reaching the South was to go back to school -- 12 years after finishing his education.
North Korea claims a 100 percent literacy rate and boasts that its free compulsory education demonstrates the superiority of its socialist system.
But those who escape from the impoverished country often struggle in the South from a lack of basic knowledge.
Lessons at North Korean schools are peppered with praise for the leadership, defectors say, and for many, education is also disrupted by grinding poverty or their long journey to freedom.
Ri, 31, is among a handful of adult students at Wooridul School in Seoul, an educational haven for North Korean students too old, or lagging academically and so unable to go to appropriate state schools.
"Although I studied in the North and graduated, I don't know much," said Ri, who went back to school last year, six months after arriving in South Korea.
Much of what he was taught in the North was not applicable in his new home, he added: "Everything I learned is different."
- Revolutionary studies -
One of the most important subjects in the North Korean education curriculum is revolutionary studies, which focuses on the ruling Kim family.
It starts with two hours a week at the age of six -- when pupils are taught the official versions of the childhoods of the country's founder Kim Il Sung and his son and successor Kim Jong Il, grandfather and father of the current leader Kim Jong Un.
Soon afterwards Kim Jong Il's mother Kim Jong Suk joins the pantheon, and in secondary school six classes a week are devoted to the subject -- a significant percentage of the total teaching.
When AFP visited Manbok high school in Sonbong, North Korea, principal Ri Myong Guk said: "Our students grow up in the love and care of the party and the state.
"We believe it's important to educate the students with political and revolutionary history so they appreciate the love and care of the great leaders," he explained.
The South Korean government describes the North's education system as designed to instil "unconditional loyalty to the party and the leader as the most important aspect of life".
And Lee Mi-yeon, a former kindergarten teacher in the North who fled in 2010, added: "They are taught as mythical, God-like figures who created the country and made grenades out of pine cones."
Teachings about the leaders seep into other subjects as well, she said.
"If we are teaching about the construction of a building, we have to spend about five minutes to tell a related story about the leader for ideological education," Lee said.
- 'Re-education is essential' -
According to defectors many young North Koreans were forced to abandon their schooling when the country's economy collapsed in the mid-1990s and a famine claimed hundreds of thousands of lives.
Lee Song-hee, a 27-year-old student at Wooridul School, said that after only four months of elementary school in the North she had to drop out to help her mother as they struggled to earn a living.
"We roamed through mountains and hills to collect herbal medicines," said Lee, who was almost illiterate when she first came to the institution in September 2017.
Some 60 students are enrolled at the school, one of seven special-purpose academies across the country, offering defectors free education that its principal says is "crucial" for life in the South.
"At the very least, re-education in culture, language, social studies and history is essential," added Yun Dong-ju.
It is an issue -- along with gaping attitudinal and social differences, among others -- that would pose fundamental challenges to any scenario of reunification between the Koreas.
In the highly-competitive South - where more than 90 percent of the population finish high school and 40 percent go on to universities - current newcomers are bound to experience huge gaps in education and skills, Yun said. But many lack even the basic education normally given at elementary and middle schools.
For school-age defectors, his institution offers a chance to catch up with South Korean cohorts at public schools -- but there are some lessons they cannot learn there.
Lee Hyung-jong, a researcher at the Institute of Far Eastern Studies, warned special purpose schools may have unwanted side effects by reducing children's interactions with South Koreans of their own age.
"School is not just a place for studying," he said, "but where a student learns how to socialise by building relations with the teachers and with their peers".