Mi brought in Lane Litz, who had worked in three countries on how children learn second languages, to lead development of a standardized curriculum so the company would have control over what was taught. The team worked on easy-to-use software that would allow students in China to learn from native English speakers a half world away with real-time audio and video links.

The software works a bit like corporate video-conferencing. The student and teacher appear in boxes on the right-hand side of the screen; images and words appear on the left. A typical 25-minute lesson has about 30 slides and the curriculum builds from session to session so a child can develop their vocabulary and fluency.

Parents buy a package of lessons and their children can then select which teachers they want. A block of 72 classes is about $1500, or about $21 each.

Douglas Gao, 10, began studying English as part of an early trial group in 2014 and has kept going. His father, Victor, says the price is comparable to a group English class his son took, but those were inconvenient and ineffective. Now Douglas is speaking in complete sentences and his six-year-old brother has started the program.

"Of course face-to-face is the most effective method for learning, but kids these days are very computer-friendly so it's very easy for them to pick it up," the father says. "On the other side of the computer are very experienced teachers. They really know how to deal with these kids and how to get their concentration."

VIPKid recruits teachers through referrals and social media and gets 10,000 to 20,000 applications a month. Prospective hires are tested and screened and about one out of every 10 applicants are approved, Mi says. Successful ones take VIPKid's training courses for about a week and need to pass a final test before starting work. They're not required to have degrees in education, but do need a bachelor's degree and some teaching experience.VIPKid says most of its instructors are experienced current or former teachers.

April Baker, a 42-year-old single mother in Pennsylvania, started teaching with VIPKid in August, in part because of financial pressures from her divorce last year. She teaches most days between 7 a.m. and 9 a.m. and often again from 8 p.m. until midnight. The $20 to $22 an hour she makes helps supplement her income as a fitness instructor for seniors, and the flexible hours allow her to look after her kids.

"It feels so freeing," says Baker, who has undergrad and graduate degrees in education and worked in the field for about 10 years. "The only issue is the time zone is a challenge."

Kristie Kellis, a 41-year-old from Minnesota, started teaching with VIPKid a few months ago. She now works 40 to 45 hours a week, usually from 3 a.m. to 8 a.m., in addition to her day job teaching at a local university. She's paid about $21 an hour as long as she works at least 20 hours a week, and then gets bonuses for teaching more hours or during holidays. She says it works out to about $4,000 a month plus bonuses.

"This rivals what I can make at a university," says Kellis, who has a bachelor's degree in child psychology from the University of Minnesota.

Both Baker and Kellis say it's refreshing to work with eager kids and supportive parents. Kellis had one six-year-old girl who was shy at first, but became comfortable speaking and reading English in two months. The key: The girl's mother worked with her on sounding out words from letters, instead of memorizing like Chinese do with characters. "Her mom was exceedingly helpful,'' Kellis says, adding that most students take five to six months to learn decent English.

The teachers also appreciate the support they get from VIPKid. The company sends out weekly videos with tips on new techniques instructors have found useful and stories of success with students. The company has a team of "firemen" who help with technology problems.

VIPKid offers regular incentives for teachers too, including computer gear and T-shirts. Kellis won a trip to Beijing next month to meet Mi, her employees and students. "They put so many of their resources back into the teachers," she says.

Mi says she hasn't begun to think about an IPO because there is so much work to do in managing growth. She thinks it may be only three or four years before VIPKid has a million students, a dizzying prospect for a three-year-old enterprise.

She sees an opportunity to improve the effectiveness of education with such an enormous group of students and she's set up a research institute to work on developing best practices. Robert Hutter, managing partner of the Silicon Valley venture firm Learn Capital, is chairman of the group, and its advisers include professors from Stanford, Harvard and the University of Southern California. "The sheer scale of this presents the opportunity to examine theories that you couldn't look at in your own lab,'' says Bruce McCandliss, a professor at Stanford's Graduate School of Education who will help develop the research agenda.