I would necessarily trust the metrics of this. Much of it is comparing apples to oranges.
For example, comparing the the huge and diverse USA to Singapore, which is a highly advanced and homogenous, but relatively small city, is not a good comparison.
Neither is comparing USA to China. China will never allow any foreigner to interview, survey or deeply analyze any part of its society. Data from China should be suspect.
Even a comparison to Germany may not be accurate, as by high school Germany has already segregated and separated kids by testing and career path.
REGARDLESS, by our OWN standards, kids in the USA are certainly worse than 40 years ago.
RE: … Singapore, which is a highly advanced and homogenous…
Not sure what you mean by homogeneous.
Singapore’s ethnic makeup is 75.5% Chinese, 15.1% Malay, 7.6% Indian, and 1.8% Eurasian/Other. Because of this diversity, Singapore is NOT considered a homogeneous country; instead, it is officially recognized as a multi-racial, multi-ethnic, and multi-cultural society.
Small, yes ( smaller than New York City in terms of population ), but homogeneous? Not really.