Who says kids are underperforming? Only people who have a vested interest in rubbishing teachers.
How can you compare one country against another; one child against another? Why make children, or even countries jump through the same hoops when they come: countries and kids, in different shapes and sizes, with different needs at different times, and are at different stages of their development?
Trotting the old lines about getting the basics right gets us nowhere. You're not advantaging every child; you're giving industry a bigger pool of candidates to dip into, and thus, a bigger group of would-be workers to reject. What all this Maths and English if there's no job, or if your job requires a different type of basics?
What the politicians and the public pronouncers don't know are the subtleties of teaching even these basics. Good English teaching is enhanced by good Music teaching, for example. A top mathematician is not much use to themselves or society if they have no morals, or if they are a bully? You can't just open children up and pour knowledge in. You must motivate them to want to learn; and really the more of one subject that you try to force in, the more they are likely to resent and resist.
Children, and countries also develop and mature at different rates, and measuring everyone at the same age is the most pointless, damaging, dangerous, ignorant thing to do. You learn nothing by it, and, if you then go on to make decisions based on these bizarre and unscientific measurements, you waste time, money and everyone's lives.
When the experts: teachers, teacher-trainers, professionals, teacher-unions - when these people speak out against the SATS, the Ofsteds, the academies and the like, the public pronouncers close their ears because it doesn't suit their arguments and their plans. They rubbish the very people that they really need to keep motivated. The best teachers will leave the profession and by the time the UK education has really fallen apart, Gove and henchmen will safely tucked up in their clubs.