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NCTL President Jennifer Davis said Tuesday that the report, while interesting, simplifies the issue of school time.

“The picture is a lot more complicated than the data reveals,” she said. “Families in South Korea, for example, spend about 10 percent of their annual income on outside tutoring, resulting in 58 percent of their students participating in those programs. A much lower percentage of U.S. students are able to access similar programs. Therefore, our country’s most disadvantaged students must rely exclusively on their time in school to get the education they need.”

South Korean students also report spending nearly five hours per week on a combination of “out-of-school” mathematics lessons, such as homework, and “independent study” not assigned by teachers. For U.S. students, it’s about three hours a week, according to PISA.

Adding more time to the school day, Ms. Davis said, would begin to level the playing field.

But Mr. Hull, the study’s sole credited author, argues that lengthening the school year, while maintaining the same curricula and teaching methods, isn’t the answer.

“Providing additional time can be an effective tool for improving students’ outcomes, but how that time is used is most important,” Mr. Hull said.