U.S. teenagers made no significant gains on an exam taken by students around the world, and continue to trail students in Asian countries.

The exam, called the Program for International Student Assessment, or PISA, is considered a barometer of future economic competitiveness and is given every three years. It covers math, reading and science and targets 15-year-olds in private and public schools.

Results released Tuesday for 2018 show a widening gap between higher- and lower-performing students in the U.S. in reading and math. And despite average scores inching up in all subjects for American students, federal education officials said they weren’t measurably different from the last testing cycle in 2015.

“The scores are flat. We’re struggling in math in comparison to our peers around the world,” said Peggy G. Carr, the associate commissioner of assessments for the National Center for Education Statistics. “We’re sliding with regard to our most struggling readers.”

A representative sample of about 600,000 students in 79 countries and education systems took the two-hour PISA exam in 2018.

China, represented by four provinces, had the highest scores in all three subjects, with an average reading score at 555, math at 591 and science at 590, on a scale of zero to 1,000.

U.S. students had an average reading score of 505, a math score of 478 and a science score of 502. Socioeconomically advantaged students in the U.S. outperformed disadvantaged students in all subjects.

The U.S. ranking improved in all three subjects to eighth in reading, 30th in math and 11th in science, when compared with 63 other educational systems that reported data in 2015 and 2018. But Ms. Carr said the improved rankings are due to score changes with other education systems.

The PISA measures how well students apply knowledge to real-world tasks as they near the end of required schooling. The main focus of the assessment rotates between math, reading and science. In 2018, reading was the major domain, with half the assessment devoted to the subject. Math and science were minor domains, with one-quarter of the exam devoted to each.

In the U.S., about 13.5% of students were good at distinguishing between fact and opinion on the reading exam, according to the Organization for Economic Cooperation and Development, a group of 37 mostly industrialized countries, including the U.S., that developed the PISA exam. That average for OECD countries is around 10%.

“In the world of, you know, fake news, it’s very important that students can actually navigate ambiguity and resolve conflicting dialogue,” said Andreas Schleicher, OECD director for education and skills.

Most countries have seen little improvement in scores over the past decade, despite increases in education spending, according to OECD.

The showing for the U.S. follows October’s flat results on a national exam for reading and math, known as the Nation’s Report Card. Scores on the exam, taken every two years by a sample of fourth- and eighth-graders, show that some of the lowest performers fell further behind, a similar pattern seen with the PISA.