Not sure if you're a sports person, but imagine if the NBA had a franchise that was never to participate in the draft at all, and everyone kept asking why they never made the playoffs.
Joking aside, I believe the answer is to reframe the question and ask what society can do. The core problem is that nearly all of our efforts to fight poverty are designed in such a way that the less poor you are, the more you're able to benefit. I believe we need to shift our methods to prioritize the people with the deepest need, at every level.
In such a society, schools like these, which catch by far the most challenging demographics, would be by far the best resourced. The goal would be to rehabilitate these most challenged students to decreasing levels of intervention over time.
Maybe ask kids what they want to study? We all know you get further with carrots than sticks.
As for income, who knows. Surely in a world where CEOs make in hours what average people make in a year there are compensation issues.
So I think you're right that the American system should seek to have less tests and especially less standardized tests, instead relying more on the pedagogical expertise of teachers. That will be hard to accomplish because it would require somehow implementing another element of the Scandinavian system: teachers have both high competency requirements and high motivation for the field.
Scandinavian school systems
that have demonstrated good results
[...] have spent decades gradually
de-emphasizing tests and grades.
What conclusion do we draw from this? One might wonder if different demographics do well with different forms of teaching.
I guess people like to focus on schools because it ties into the idea that we're a meritocracy ("anyone will succeed, as long as they go to a decent school"). It also lessens the burden for the rest of society - we only need to think about making schools decent enough. We don't need to think about putting more resources into supporting struggling families, or reforming societal structures at the root of massive inequality.
After a few years of living in downtown Baltimore and similar places, I've concluded that equality of opportunity achieved through schools is a crock. My wife and I lived in a high income Baltimore neighborhood (Bolton Hill) adjacent to a very low income one (Sandtown). Our daughter went to a private pre-K. You could put a retinue of Nobel laureates on the faculty of the elementary school in Sandtown and you wouldn't be able to convince the folks in our neighborhood to send our kids there. Trying to fix society with school is like trying to wag a dog by its tail. We're talking about neighborhoods here with massive dysfunction. Where gangs literally divide up the blocks and you'd better be in one or another lest you get beat up walking to school. There are no jobs--you see men milling around the neighborhood in the middle of the day. Teachers can't fix that.
For one thing, we project the merit of parents (or lack thereof) onto their children. We do this in how we allow massive intergenerational transfers of wealth. But also, as you point out, the schools a child can attend are very dependent on who their parents are.
In Chris Hayes' excellent book, Twilight of the Elites [1], he rejects meritocracy. Not only does he reject it in practice, for reasons like those I presented, but he also rejects it even in theory. He suggests that the idea of showering people with resources that show promise is an affront to the concept that every human being has fundamental value and that all should be invested in.
Liberals, broadly speaking, do love schools. But I don't think that's for lack of concern about other issues of social justice! Perhaps you mean economic liberals (e.g. classical liberals and neoliberals)?
[1] http://www.amazon.com/Twilight-Elites-America-After-Meritocr...
Actually, liberals are far more likely to focus on the broader social justice issues with the schools as one component. It's conservatives who tend to focus on school performance in isolation (with the usual remedy being various forms of privatization.)
edit: didn't realise who I was talking to, more respect is due (or less opining on 'baltimore' and 'teaching'). I've read some of your journal.