You can find out the answers to the factual questions you raise (not from anecdotes, but from published, peer-reviewed research reports) by looking up the publications of the two main international educational studies, TIMSS and PISA.
Simply put, you are mistaken if you think that the United States is especially disadvantaged by how the national sample surveys on which those programs are based are constructed.
AFTER EDIT: Following up on the two replies now shown, the PISA FAQ document does not imply that there is any systematic different in national sample characteristics by learning disability, which is the claim farther up in this subthread. Further, people who have read the extensive research literature on learning disability identification in varying countries will be well aware that the United States is particularly generous in identifying students as having "special needs" as compared to other countries, so a general student sample from other countries would be more, not less, likely than a United States sample to include unidentified students who have personal special learning needs.
Okay, where in those studies do they mention which students were sampled? I cant find anything. I highly doubt Hong Kong, Singapore and China include children with disabilities in their tests.
http://timss.bc.edu/
http://nces.ed.gov/timss/
http://www.oecd.org/pisa/
Simply put, you are mistaken if you think that the United States is especially disadvantaged by how the national sample surveys on which those programs are based are constructed.
AFTER EDIT: Following up on the two replies now shown, the PISA FAQ document does not imply that there is any systematic different in national sample characteristics by learning disability, which is the claim farther up in this subthread. Further, people who have read the extensive research literature on learning disability identification in varying countries will be well aware that the United States is particularly generous in identifying students as having "special needs" as compared to other countries, so a general student sample from other countries would be more, not less, likely than a United States sample to include unidentified students who have personal special learning needs.