The United States may be struggling right now, but China has significant, major challenges they need to address before they can continue to to grow.
For one, China's production of food is not sustainable given their massive population. There has already been widespread reports of overtaxing of aquifers, and there's undeniable evidence already of the rapid expansion of the Gobi Desert as a result of overgrazing, deforestation, and other human impact. That's not to say that the United States hasn't significantly impacted the environment of our own lands, but the simple fact remains that, as 1.3 billion people become more prosperous, so too does their energy expenditure increase.
For second, let's not beat around the bush here. China is hiding some major debt problems of their own. When the world economy hit the shitter in 2008, China, like the United States, bailed itself out. But imagine if we spent $4 trillion bailing ourselves out, and that's essentially what China did if you compare it to a percentage of their GDP. Unless China makes significant reduction in its overall debt levels, their economy will go the way Japan's did; level off.
For third, the United States had an overabundance of one specific resource that played an extremely strong contributing role to the overall economic prosperity of our nation during the 20th century; oil. China, unfortunately, will not have this [cheap] luxury during their rise in the 21st century (a luxury that isn't cheap for us anymore either).
And that leads me to this; to think that the United State is simply going to crash and burn only to be a page in the world history book, is kinda silly. Great Britain was once a world superpower. France was once a world superpower. Russia was just recently a world superpower. Sure, they may not enjoy that status like they once did, but that doesn't mean their land a smoking ruin of has-been's. The term "superpower" is simply a relative term. China, if they're able to work beyond their challenges, may supersede the United States as the world's leading economic superpower, and maybe if they steal enough of our shit, a military superpower as well, but that doesn't mean our nation is in decline.
The 21st century will be the defining moment for mankind as we begin to deplete our more easily accessible natural resources. No nation, not China, not the United States, not Russia, and not South Africa, will be free of the challenges we'll all eventually face. But the United States has weathered through far worse crises, and I'm sure our nation of intelligent people will be just fine once the "Carbon Era" of mankind is over.
Lastly, you can't directly compare test scores between nations with differing education policies. Asian nations typically employ more rote-based education systems, whereas European and American education focuses more critical thinking. Rote educated nations do typically test more favorably than those in the west, but they're still 100 years behind us to walk on the moon.