Kim Ki-hoon has risen to stardom of a sort that exists in few places outside South Korea. As the country's highest-earning celebrity English teacher, he estimates he made about $4 million last year from his online language lessons – and then there is the income from his educational publishing company, which turned over about $10 million.
The star tutor says about 1.5 million South Koreans have taken his classes in the past 12 years. He attributes his success to his engaging teaching style and clever marketing: he selects his television appearances carefully, and made a pop song aimed at nervous university candidates with a chorus urging "Trust me!".
Mr Kim's earnings are a fraction of the estimated $20 billion spent annually on private tuition in South Korea. Yet he, like others in the industry, expresses unease at the scale of the system that paid for his Porsche. "There should be no need for private education," he says.
The fierce debate in South Korea over the national education system – in particular, the huge industry of private crammers (hagwons) and online study providers – might surprise its foreign admirers. Figures such as US President Barack Obama and Michael Gove, UK education secretary, have hailed South Korean education as a model, pointing to its children's impressive showing in Organization for Economic Co-operation and Development tests of maths, literacy and science.
Within South Korea, however, there is growing unease at the side-effects of the massive use of private education. For a start, many children study from early until late after the end of the regular school day, leaving little time for play or relaxation.
Last year, a study of children in the wealthy Seoul district of Seocho-gu found that almost one in seven suffered from curvature of the spine, more than double the rate 10 years before. At least three quarters of Seoul's high-school pupils are myopic, and physicians are spotting more and more youngsters with "turtleneck syndrome", in which the child's head hunches forward anxiously.
Then there is the economic impact: private education costs account for about 12 per cent of total household expenditure and are widely blamed for depressing South Korea's birth rate, one of the lowest in the world.
Successive governments have tried to rein in the thousands of hagwons. They are now banned from operating after 10pm, and special inspectors can raid any suspected of breaking the rule. Hourly tuition rates have been capped and, most recently, hagwons were banned from teaching material ahead of it being studied in school.
The new rules are anathema to Min Seong-won, the country's most famous education consultant, who says: "South Korea's education system is becoming like a socialist system."
Mr Min operates a hagwon, and dispenses advice on education planning through television appearances and a string of popular books. His unique offering is the "road map" – a bespoke service that includes him devising a detailed plan for getting a child into college years in advance.
After reforms to the admission process, universities can assess applicants on a range of factors from school grades, entrance exams, English tests and extracurricular pursuits. But the constantly changing system has bewildered parents. "There are 3,000 ways to get into Seoul National University," says Mr Min, wielding a dense book of statistics to prove it.
He points to the country's showing in the OECD's Pisa tests as evidence of the education system's strength. In the 2012 tests, taken by 15-year-olds in 64 territories, South Korea came fifth in maths and literacy, and seventh in science. Yet the same test put South Korean children 62nd in confidence in solving complex tasks, and last for happiness at school.
Lee Ju-ho, education minister from 2010-2013, believes this indicates the impact of excessive study on mental health, creativity and teamwork skills, and the need to "combat the private tutoring business". If South Korea is to flourish, he says it must address the problem of "high expenditure on education that is not leading to an increase in human capital".
He points in particular to private English tuition, where spending is equivalent to 0.6 per cent of gross domestic product, but where the results fail to match this multi-billion-dollar outlay and confident speakers of English are rare.
Many, including the hagwon teachers, blame the means used to assess English aptitude, which are overwhelmingly multiple choice comprehension exams, such as the Test of English for International Communication administered by the US-based group ETS.
Yoo Su-yeon's success in preparing students for the TOEIC test has made her a TV celebrity who earns $1.5 million a year teaching. But for Ms Yoo, the TOEIC syllabus is "not helpful" for a good understanding of English. "It's not really an English test – it's a way of identifying who has basic ability, and who wants to learn in their new job," she says at her hagwon, U-Star, which employs 40 or so teachers. "Companies choose TOEIC because it's about following instructions."
Many South Koreans are intensely proud of the role education has played in the economic growth of a country that in 1945 was among the world's poorest, with a literacy rate of 22 per cent. While acknowledging the system's shortfalls, some business leaders say it does a good job of providing skilled workers for cutting-edge industries. "I think we can do better," says Sirgoo Lee, chief executive of Kakao, the mobile chat app company. "But this really strong interest in education, and this very fierce competition in education, creates a top-notch, very highly talented pool of young folks."