Too often, it's very safe to say that most people who talk about or even use test scores as a litmus test for schools really have no idea what these tests really look like, how they are made or even the real variables that dictate the scores.
In his article Common Core Standards: I wonder?, Christoper Tienken writes:
Another factor that impacts international assessment scores is that participating countries negotiate the test items that will be included. Representatives from more than 60 countries negotiated the development, wording, skills, and context of the items in the PISA 2003 assessment. The United States does not do well in these negotiations, as reflected by the fact that there are typically more test items covered by Asian curricula than by typical U.S. curricula. The Asian countries have reputations for scoring high on international tests, and there is intense national pressure to maintain that reputation. Their negotiating teams do their best to ensure that trend continues. Also, their countries produce test preparation materials based on skills they know will be included. For example, PISA and TIMSS test preparation materials are readily available in stores in Taiwan and Singapore (Sjøberg 2007). Thankfully, I have not yet seen any prep materials for these meaningless tests on the store shelves in the United States.