Clasping his headphones and closing his eyes as he sang into the studio microphone while performing a peppy duet with one of South Korea’s hottest actresses, spiky-haired Cha Kil-yong looked every bit the K-pop star.

But Cha is not a singer or actor. No, he’s a unique kind of South Korean celebrity: a teaching star. And the song he was singing with Clara, a Korean mega-celebrity, in an MTV-style music video? It was called “SAT jackpot!”

In this education-obsessed country, Cha is a top-ranked maths teacher. But he doesn’t teach in a school. He runs an online hagwon – or cram school – called SevenEdu that focuses entirely on preparing students to take the SAT-equivalent college entrance exam in mathematics.

Teaching pays: Cha said he earned $8m last year. “I’m madly in love with maths,” he said, looking the height of trendiness in his crimson shirt and trousers and tweed jacket, in his office in Gangnam – a wealthy part of Seoul famous for its conspicuous consumption and featured in the song Gangnam Style.

It’s hard to exaggerate the premium South Korea places on education. This is a society in which you have to get into the right kindergarten, so that you can get into the right elementary school, then into the right middle school and high school, and finally into the right college. Which, of course, gets you the right job and scores you the right spouse.

There’s even a phrase to describe the Korean version of a helicopter mother: chima baram – literally skirt wind, to describe the swish as a mother rushes into the classroom to demand a front-row seat for her child or to question grades.

Many Korean families split and live on opposite sides of the world in pursuit of a better education: the mother and children live in the United States or another English-speaking country, the better to secure entry to a prestigious university (preferably Harvard). The “goose father” continues working in South Korea, flying in to visit when he can.

All of this combines to make South Korea’s equivalent of the SAT the most important event in a young person’s life.

The vast majority of teenagers attend normal classes by day but go to hagwons for after-hours study. Increasingly, online hagwons are replacing traditional brick-and-mortar cram schools. The hagwons have become a $20bn industry.

Facebook Twitter Pinterest Masks and props used by Cha in his SevenEdu filming studio in Seoul.

This devotion to studying is credited with helping South Korea rank at the top of the developed world in reading, maths and science, although the latest rankings [PDF] from the Organisation for Economic Cooperation and Development also show that Korean students come last when asked whether they are happy at school. South Korea also has the highest suicide rate in the developed world, which many suggest is related to a high-stress focus on test scores.

Some politicians and educators are questioning whether things have got out of hand. But even parents opposed to this punishing system find it difficult to opt out – their children complain that they can’t keep up if they don’t go to a hagwon.

That’s good news for instructors like Cha, who started teaching at a hagwon to pay his way through his PhD programme.

About 300,000 students take his online class at any given time, paying $39 for a 20-hour course (traditional cram schools charge as much as $600 for a course). He teaches them tricks for taking the timed exams, including shortcuts that students can take to solve a problem faster.

Asked what makes him stand out as an instructor, Cha said: “Suppose you give the same ingredients to 100 different chefs. They would make different dishes even though they’re working with the same ingredients. It’s the same with a maths class. Even though it’s all maths and all in Korean, you can use different ingredients to come up with different results.”

His studio is set up with a green chalkboard and desks, and behind the camera are piles of props – including hippo and Batman masks and a gold-sequined jacket.

“You’re not only teaching a subject, you also have to be a multi-talented entertainer,” said Cha, declining to give his age and offering only that he’d been working for 20 years.

On SAT day, he visits schools to offer encouragement to test takers. He also does television ads, endorsing products such as a red ginseng drink meant to boost brain power.

Kwon Kyu-ho, a top-ranked literature teacher, also appears with K-pop stars and has a lucrative side business in celebrity endorsements, lending his name to a chair meant to help people study better.

Maintaining his position doesn’t require just good lessons. Kwon, 33, also gets regular facials and works out, and he said some teachers even have stylists.

“I always wanted to be a teacher, but I feel that regular school teaching has its limits. There is a certain way you have to teach,” said Kwon, whose lessons appear on the sites Etoos and VitaEdu. “And, of course, I’m making a lot more money this way.”

He wouldn’t disclose how much he earned, only that it was “several millions” of dollars a year. The secret of his success, Kwon said, was finding the parts of tests that make most students stumble. He focuses lessons on those problem areas.

This style of education has its upsides, he said.

“I think one of the benefits of private education is that teachers compete with each other and try to develop higher quality content,” he said. “We have money. We can invest in ways that normal schoolteachers cannot.”

As President Park Geun-hye promotes a “creative economy” as the key to taking South Korea to the next level in its development, many analysts say the country would do well to take a more creative approach to education.

Lee Ju-ho, former minister of education, is among them.

“All this late-night study could lead to problems in enhancing their other skills, like character, creativity and critical thinking,” he said. “Hagwon is all about rote learning and memorisation.”

Lee said all the problems stem from the college admissions procedures, which have been slow in looking beyond test scores to other criteria such as extracurricular activities and personal essays, as is common in many western countries.

“We really need to change,” said Lee, who is now a professor at the Korea Development Institute’s School of Public Policy and Management.

This article appeared in Guardian Weekly, which incorporates material from the Washington Post