First, I was specifically responding to the “abysmal” characterization — I think there's plenty of room for improvement. The main point was that when making comparisons across countries we have to perform some corrections if we're looking for ideas about where we could improve. Since wealth isn't evenly distributed across countries or within them it's easy to find a smaller country which looks like an outlier, run some editorials about how they've discovered the secret to education, and not really have learned anything other than that life easier if you're not poor.

What we'd want to look for in setting goals are the countries with high social mobility because, as you mentioned, it's better when students can do markedly better than their parents. The results of that comparison likely also ideas outside of the educational system itself: for example, if the child of poor immigrants in Scandinavia does better it might be that their teachers and curriculum are about the same but the better social support system means their parents aren't working 3 jobs to make rent or asking their oldest child to stay home to watch their siblings while they work.

Part of why I mentioned immigrants in that previous example is that this is also a complicating factor for the U.S. because we have a relatively large number of immigrants compared to many of our peers and a large fraction don't arrive speaking English. Many older children score poorly that way due to language proficiency, so using those figures to attack the educational system is a disservice to both sides.