UK education lagging behind the world's best?

I invite you to try this sample of the Pisa maths test at http://www.oecd.org/pisa/test/

The tests and results are interesting, for several reasons.

The questions are surely ridiculously easy for a 15-year-old, even at the highest test level.

Sub-common entrance standard? At this level of intellectual challenge, errors due to carelessness may be as common as errors due to incompetence.

As a likely consequence, some 29 countries score at or above 90%, which might put the differences above that score firmly into the zone of statistical error.

I still don't understand the sampling methodology, and as any statistician will say, the main threat to proper inference is the risk of sample bias. Is it really possible to use a common sample frame across so many disparate communities and cultures? 

Shanghai may score highly among the kids that go to school. But what about the ones that don't?

In other words the Shanghai results may be better compared with those from UK private school students.

I have no idea how our educations standards really compare internationally, and I remain uninformed.