"Personalized learning" is one of the trendiest educational theories in Silicon Valley right now.

It involves each student learning at his or her own pace, generally through the aid of technology, and it's beloved by tech billionaires like Bill Gates and Mark Zuckerberg.

But both Gates and Zuckerberg, in addition to other big names in Silicon Valley, also back an education model that is taking the opposite approach: pursuing learning gains with curricula standardized to each particular country.

Bridge International Academies, the subject of a recent New York Times Magazine piece, operates in hundreds of schools around Kenya and Uganda, with dozens more of its low-cost schools scattered through Nigeria, India, and most recently Liberia.

Though it operates with the mission of providing high-quality, low-cost education for all, Bridge has drawn criticism from some education experts and teachers unions for the model it uses to make good on that mission.

All Bridge students in a given country receive the same education at the same time -- something personalized learning advocates generally shun as inefficient or, worse, ineffective.

By 2015, Bridge had raised $100 million, the Wall Street Journal reported. The money came in part from Gates and the Chan Zuckerberg Initiative, a company formed by Zuckerberg and his wife Priscilla Chan to give away 99% of their Facebook shares in order to advance science and education. In addition, the Omidyar Network, the World Bank, venture capital firms, and the hedge fund Pershing Square have jumped onboard.

But even Gates, a vocal advocate for uplifting those in the developing world, has said the old-school model of education doesn't always cut it.

"It's amazing how little the typical classroom has changed over the years," the former Microsoft CEO wrote on his blog, Gates Notes. For centuries, teachers all over the world have lectured at the front of the room to students seated in rows.

"This system was designed decades ago," Gates continued, "and it doesn't reflect what educators have learned about helping students and teachers do their best work," namely, that students learn far more effectively when their instruction is tailored to their specific needs. (Gates was not available for comment on this story.)

The personalized learning philosophy underpinned special-education when it began more than 40 years ago, and it continues to work its way through in public schools around the US. Zuckerberg, too, has endorsed personalized learning time and again as the superior approach to instruction.

The fact Gates and Zuckerberg support Bridge financially likely reflects their primary goal of lifting the poor out of poverty. Smaller details about the specific style are perhaps immaterial if the data show the method is working. Bridge has data to back it up.

In 2015, the company published a white paper that found among 2,737 students in kindergarten, first, and second grades, Bridge kids produced twice and sometimes three times the gains in reading and math compared to kids in public school.

Critics point to a host of other facts as drawbacks, however. Cost is the first and perhaps highest barrier, since Bridge calls on parents to pay for tuition, lunch, and, in certain cases, school supplies. The school network has high attrition rates and numerous cases of families defaulting or falling behind on tuition payments.

Bridge founder Shannon May told the New York Times Magazine that her company is considering partnerships with microfinancing firms to help families defray the costs.

An advocate for a local teachers union also expressed concern to the New York Times Magazine about the quality of the education, arguing Bridge focuses less on getting poor students to the baseline as enticing public school students to switch to Bridge schools.

In December of 2016, secretary-general Wilson Sossion and the Kenya National Union of Teachers (Knut) published a negative report of Bridge, saying the company was operating an illegal for-profit business. Earlier this June, Bridge sued Knut and Sossion for defamation.

But true to the Silicon Valley mindset praised by people like Gates and Zuckerberg, the model seems to be filling a genuine market need. Bridge is on track to educate 4.1 million kids by 2022, with yearly revenue of $470 million.

Correction: This article has been updated to more accurately reflect how many teachers are certified to teach in Bridge schools.

This post originally appeared on Business Insider.