If the 12th graders of all 41 countries whose 8th graders participated in TIMSS had also participated in TIMSS, only South Africa would have scored lower than the US.
Of the 41 countries whose 8th graders participated in TIMSS, the 12th graders of only 18 of those countries participated in TIMSS. Of the 13 countries whose 8th graders scored lower than the US, the 12th graders of 5 of them (Greece, Cyprus, Latvia, Lithuania, and Iceland) scored significantly higher than US 12th graders, the 12th graders of only one (South Africa) scored lower, and 7 of them didn't participate in TIMSS. South Africa's 12th graders scored higher than their 8th graders, whereas our 12th graders scored significantly lower than our 8th graders in most subjects. It is probable that 6 out of the 8 who didn't participate in 12th grade TIMSS would also have scored higher than the US.
Of the 27 countries whose 8th graders scored higher than ours, the 12th graders of 13 of them participated and 14 didn't. Every one of the 13 countries whose 12th graders did participate scored higher than both boys and girls from the US in Physics, Mechanics, and Geometry. All of their boys scored higher in Electricity & Magnetism, Wave Phenomena, Heat, and Advanced Math. Only Austrian girls tied with our girls for dead last in Electricity & Magnetism, only Czech girls scored lower in Wave Phenomena, and the girls of most countries scored higher in Wave Phenomena and Heat. Only Austria, Italy, Czech Republic, and Germany scored lower in Numbers & Equations and Calculus. Only Cyprus and Czech Republic scored lower in Modern Physics. Only Cyprus, Czech Republic, and South Africa scored lower in General Math. Only Cyprus, Hungary, Lithuania, and South Africa scored lower than both our boys and girls in General Science.
All of the 14 countries whose 8th graders participated but whose 12th graders did not, scored significantly higher than the US. Singapore scored 143 points higher, Korea scored 107 points higher, Japan scored 105 points higher, Hong Kong scored 88 points higher, and it is impossible that their 12th grade scores could have been lower than ours. The lowest scoring countries whose 8th graders participated but whose 12th graders didn't were England (6 points higher), New Zealand (8 points higher), Thailand (22 points higher), Ireland (27 points higher), and unless their 12th grade scores would have been more than 100 points lower than their 8th grade scores, all of these countries would have scored higher than the US had their 12th graders participated in TIMSS.
US 12th grade girls DEAD LAST in Physics
US 12th grade girls DEAD LAST in Mechanics
US 12th grade girls tied with Austria for DEAD LAST in Electricity & Magnetism
US 12th grade boys DEAD LAST in Heat
US 12th grade girls DEAD LAST in Geometry
US 12th grade girls fifth from last in Numbers & Equations
US 12th grade girls third from last in Calculus
US 12th grade boys DEAD LAST in Advanced Mathematics
US 12th grade boys DEAD LAST in Wave Phenomena
US 12th grade girls fourth from last in Modern Physics
US 12th grade boys third from last in General Mathematics
US 12th grade boys fifth from last in General Science