I have been critical of the focus on international tests because real life teaches us that the test scores of 15-year-old students do not predict future economic success for nations. I find it bizarre that people say that America is a great country but its schools are no good. That doesn’t make sense.

Adam Grant, a contributing opinion writer for the New York Times, injects a dose of common sense into that newspaper’s education coverage:

He writes:

Which country has the best education system? Since 2000, every three years, 15-year-olds in dozens of countries have taken the Program for International Student Assessment — a standardized test of math, reading and science skills. On the inaugural test, which focused on reading, the top country came as a big surprise: tiny Finland. Finnish students claimed victory again in 2003 (when the focus was on math) and 2006 (when it was on science), all while spending about the same time on homework per week as the typical teenager in Shanghai does in a single day.

Just over a decade later, Europe had a new champion. Here, too, it wasn’t one of the usual suspects — not a big, wealthy country like Germany or Britain but the small underdog nation of Estonia. Since that time, experts have been searching for the secrets behind these countries’ educational excellence. They recently found one right here in the United States.

In North Carolina, economists examined data on several million elementary school students. They discovered a common pattern across about 7,000 classrooms that achieved significant gains in math and reading performance.

Those students didn’t have better teachers. They just happened to have the same teacher at least twice in different grades. A separate team of economists replicated the study with nearly a million elementary and middle schoolers in Indiana — and found the same results.

Every child has hidden potential. It’s easy to spot the ones who are already sparkling, but many students are uncut gems. When teachers stay with their students longer, they can see beyond the surface and recognize the brilliance beneath.

Instead of teaching a new cohort of students each year, teachers who practice “looping” move up a grade or more with their students. It can be a powerful tool. And unlike many other educational reforms, looping doesn’t cost a dime.

With more time to get to know each student personally, teachers gain a deeper grasp of the kids’ strengths and challenges. The teachers have more opportunities to tailor their instructional and emotional support to help all the students in the class reach their potential. They’re able to identify growth not only in peaks reached, but also in obstacles overcome. The nuanced knowledge they acquire about each student isn’t lost in the handoff to the next year’s teacher.

Finland and Estonia go even further. In both countries, it’s common for elementary schoolers to have the same teacher not just two years in a row but sometimes for up to six straight years. Instead of specializing just in their subjects, teachers also get to specialize in their students. Their role evolves from instructor to coach and mentor.

It didn’t occur to me until I read the research, but I was lucky to benefit from looping. My middle school piloted a program to keep students with the same two core teachers for all three years. When I struggled with spatial visualization in math, Mrs. Bohland didn’t question my aptitude. Having seen me ace a year of algebra, she knew I was an abstract thinker and taught me to use equations to identify the dimensions of shapes before drawing them in 3D. And after a few years of observing what fired me up in social studies and the humanities, Mrs. Minninger knew my interests well. She saw a common theme in my passions for analyzing character development in Greek mythology and anticipating counterarguments in mock trial — and suggested doing my year-end project on psychology. Thank you, Mama Minnie.

Most parents see the benefit of keeping their kids with the same coaches in sports and music for more than a year. Yet the American education system fails to do this with teachers, the most important coaches of all. Critics have long worried that following their students through a range of grades will prevent teachers from developing specialized skills appropriate to specific grade levels. Parents fret about rolling the dice on the same teacher more than once. What if my kid gets stuck with Mr. Snape or Miss Viola Swamp? But in the data, looping actually had the greatest upsides for less effective teachers — and lower-achieving students. Building an extended relationship gave them the opportunity to grow together.

The Finnish and Estonian education systems are far from perfect, and Finland’s PISA scores have dipped a bit in recent years. But both countries have done more than just achieve high rates of high performers — they’ve achieved some of the world’s lowest rates of low performers, with remarkably small performance gaps between schools and between richer and poorer students. Being disadvantaged is less of a disadvantage in Finland and Estonia than almost anywhere else.

Looping isn’t the only practice that makes a difference. Both Finland and Estonia have professionalized education systems — they often require master’s degrees for teachers, training them in evidence-based education practices and methods for interpreting ongoing research in the field. And teachers are entrusted with a great deal of autonomy. Whereas American kindergarten has become more like first grade, with more emphasis on spelling, writing and math, Finland and Estonia make learning fun with a play-based curriculum. Elementary schoolers typically get 15 minutes of recess for every 45 minutes of instruction. Teachers don’t have to waste time teaching to the test. And over the years, if students start to struggle, instead of labeling them as remedial or forcing them to repeat grades, schools in both countries offer early interventions focused on individual tutoring and extra support. That helps students get up to speed without being pulled off track.

Over the years, American students have consistently lagged behind two to three dozen countries on the PISA. A major factor in our lackluster results is the huge gap between our highest- and lowest-performing students. The U.S. education system is built around a culture of winner take all. Students who win the wealth lottery get to attend the best schools with the best teachers. Those who win the intelligence lottery may get to enroll in gifted-and-talented programs.

Great education systems create cultures of opportunity for all. They don’t settle for no child left behind; they strive to help every child get ahead. As the education expert Pasi Sahlberg writes, success is when “all students perform beyond expectations.” Finnish and Estonian schools don’t invest just in students who show early signs of high ability — they invest in every student regardless of apparent ability. And there are few better ways to do that than to keep students with teachers who have the time to get to know their abilities.