LONDON — Like a school principal handing out a clutch of C grades, Andreas Schleicher unveiled the results from the latest round of the Program for International Student Assessment tests last week.

For Britain, the United States and most of Western Europe, the results ranged from “average” to “poor.” British students, for example, scored exactly average in mathematics and slightly above average in reading and science. French students were slightly below average in science and slightly above in reading and mathematics. The United States were below average in mathematics and science but slightly above in reading.

For Asian countries, the news was much more encouraging, with students from Shanghai topping the chart by a considerable margin, but with students from Singapore, Hong Kong, Taiwan and South Korea all closely bunched at the high end.

Mr. Schleicher, the head of education at the Organization for Economic Cooperation and Development, which administers the tests every three years to about half a million 15-year-olds in 65 countries around the world, also noted significant improvement in Vietnam. He described it as a poor country whose students outperformed peers from many wealthier nations — and did even better once differences in income were taken into account.