American 15-year-olds continue to turn in flat results in a test that measures students' proficiency in reading, math and science worldwide, failing to crack the global top 20.

The Program for International Student Assessment, or PISA, collects test results from 65 countries for its rankings, which come out every three years. The latest results, from 2012, show that U.S. students ranked below average in math among the world's most-developed countries. They were close to average in science and reading.