I found the quiz to be pretty interesting. I'd venture to say that most if not all high school students in Japan would ace it except for the questions asking about "preferred" American and British spellings.
If you take the other tests on the page: http://www.businesswriting.com/tests/index2.htm then you will see how tough they are. Luckily, I got 94% on the word pairs test; phew. I would say that the common usage test is pretty much designed to give you a low score, because they bundle several answers into one question without marking it fractionally, also some of the 'rules' are fairly arbitrary (like the double- or single-quote usage rule, for example. According to that test my " 'rules' " usage is wrong.).