Why the gap?

In Bob Herbert's column "Clueless in America" there is the following quote:

Mr. Golston noted that the performance of American students, when compared with their peers in other countries, tends to grow increasingly dismal as they move through the higher grades:

“In math and science, for example, our fourth graders are among the top students globally. By roughly eighth grade, they’re in the middle of the pack. And by the 12th grade, U.S. students are scoring generally near the bottom of all industrialized countries.”

What causes this gap in performance?