SEOUL, SOUTH KOREA: For a teacher who never sees his students and instructs only online, South Korean Woo Hyeong-cheol makes a lot of money, $4 million a year to be exact.

Woo is not affiliated with any institution that is part of the official school system, but the 46-year-old math instructor is considered one of the best cram school tutors in education-obsessed South Korea, with his web-based classes as well-known among test-taking teens as top-rated TV dramas.

"School teachers are concerned about creating moral people. We focus more on getting the students better grades in a short amount of time. That's why we are needed and popular," said Woo, who commands a salary higher than almost all of the top baseball players in the country's professional league.

Woo is among a group of about a dozen instructors raking it in because they are thought to be the best at raising scores.

The bulk of their income comes from online classes that are easily accessible in the world's most-wired country, where more than 90 percent of households can receive high-speed Internet.

South Korean teens are often in the classroom for at least 10 to 12 hours a day, preparing for entrance exams that determine whether they will enter a top university, which in turn can lead to an elite career path and which can even make them a good catch when the time comes to get married.

Last year, about three out of four students received some form of private education after school hours. The money spent on cram schools and tutors hit 20.9 trillion won ($16.33 billion), according to the National Statistical Office.

Woo, dubbed "the shovel" for his threats to whack unruly teens with big metal objects, posts tutorials on the Internet, where they are accessed by about 50,000 paying subscribers.

He combines straight talk, humour, intimidation and most importantly, guidance on how to find correct answers to the difficult math section of entrance exams.

"I think I can trust him up until the big day," one of his student said in an anonymous Internet posting.

Brand Name teachers

Online classes, far cheaper than cram schools, have become a standard part of education expenses for parents, who also usually hire private tutors and enrol their children in cram school.

Critics say the system is geared toward passing the entrance exams, which means students have few analytical skills. In exam subject such as English, students are much better at answering written questions about grammar than speaking the language.

The pressure-packed education system is also blamed for making South Korea have the lowest birthrate in the developed world. It has led some to simply opt out, with fathers, typically the breadwinners, living cheaply in South Korea to pay the costs of sending the children, mothers in tow, overseas for education.

"These late night classes ... can lead to various problems, including a lack of sleep that decreases the effectiveness of learning while raising issues of mental health," said Woo Ok-yeong from Health Education Forum, a child advocacy group.

In her miniskirt, boots and fashionable tops, Rose Lee looks more like a university student, but she's actually one of the country's highest paid English teachers.

Calling herself the "Queen of English" but who asked to be interviewed in Korean, Lee expects to make more than $7 million a year mostly through online classes. She also works offline, which in the cram school trade means teaching students in a classroom.

"Due to the lack of resources in our country, parents have always felt that education was the best thing they could provide for their children," Lee said through a translator.

Lee has not had much time to enjoy her wealth and knows that her fortunes can easily change in a world where she is dependent on the approval of fickle teenagers.

"I guess the parties will have to come after my retirement."