Trump has claimed American schools are worse than those in 'third-world countries' - here's why he'll probably continue to do so

Throughout his campaign, President-elect Trump repeatedly disparaged what he saw as America's failing school system.

"We are rated 28 in the world. The United States, think of it, 28 in the world," he said in a video he uploaded to Facebook earlier this year. "Third-world countries are ahead of us."

Though he never explicitly mentioned the source of that rating, it is likely that he was referring to the international test most widely used as a tool for measuring education systems worldwide, the Program for International Student Assessment, or PISA.

And while "third world" is a vague descriptor that means different things to different people, it's likely he meant developing or poor countries. One country that continues to beat the US on the PISA is Vietnam, where the average income of a citizen is $5,070 yearly, compared with $53,470 for the US.

The results of the 2015 exam, which dropped on Tuesday, again support Trump's claims. The US remained flat in reading and science scores, but declined 11 points in average math score, once again raising the question of global competitiveness of the US educational system.

When looking at a comparable sample of countries that participated in the PISA exam in both 2012 (the last time the test was administered) and 2015, the US ranking fell to 35th from 28th in math.

And Vietnam outperformed the US in both math and science.

The PISA exam is a worldwide study administered every three years by the Organization for Economic Co-operation and Development (OECD), and it measures 15-year-olds from different countries in math, science, and reading.

While the results seem disheartening to Americans, there are some factors that should provide some optimism for the US, according to Jon Schnur, executive chairman of America Achieves.

The US decline in math scores was not statistically different from the drop in scores among all OECD countries, Schnur said on a press call on Tuesday, and the US was No. 1 in the world in closing the science achievement gap between wealthy and disadvantaged students.

Still, Schnur said, the US needs to make dramatic progress in showing educational improvement for students.