It's difficult to draw meaningful conclusions from country-level differences like this, especially when the countries are as dissimilar as the U.S. (pop. 340M) and Singapore (pop. 4M).

Looking specifically at PISA, it's not usually administered at the state level in the US, but when it has, individual states have outperformed national scores, as one should expect.

For example, Massachusetts has scored similarly to Singapore in Reading and Science (zero or small statistical difference between) and not far in Math.[1] It would be a reasonable hypothesis that a PISA score for the Greater Boston schools (pop. 5M) would even further outperform the U.S.

Sweeping country comparisons tend to amplify noise rather than reveal a clear signal, and are often about regression to the mean as much as anything else. It's not impossible to make sound inferences, but it's difficult to avoid motivated reasoning.

1. https://www.doe.mass.edu/news/news.aspx?id=24050

It’s not clear to me PISA scores mean anything, because they’re not uniformly administered in the U.S. And if the scores are reliable, they point to distributional issues outside the educational system. For example, Asians in the U.S. perform comparably to Singaporeans: https://www.edwardconard.com/macro-roundup/us-asians-scored-.... White Americans perform comparably to Japan and South Korea, and ahead of all of Europe.

Education is the purview of the States, not the Federal Gov. So if you're comparing scores you have to compare Singapore against individual States. You will find a HUGE disparity between states. Some do quite well, close to SG levels. Others are abysmal.