If a child is encouraged to work hard and put in effort (this is where Tiger Moms may be onto something), they are going to achieve regardless of socio-economic status, in sports look at the Williams sisters for example. If a school fosters a culture of excellence through work and effort, students will succeed. Teacher-led education, with the right motivation can achieve this, as can a system that is more child-led. Success is a mindset.
They may achieve regardless of socio-economic status, but the idea that a child can learn anything with enough effort and a culture of excellence still won't work out with every child succeeding. There still need to be acknowledgement of individual strengths and weaknesses. If most or all children in a class are achieving excellence, than the content is too easy. There is going to be a large portion of the class that isn't even trying or learning anything. It's one of my issues with the grading system in the US and Canada, people are so obsessed with A's, even a B is considered by many as not being good enough. Yet the system is based on the bell curve. For all intents and purposes of the letter grading system most students should be getting C's, which is average, where as only those who have already learned the subject or excel at the subject should be getting A's and B's.
The problem with the system isn't that there is no expectation of excellence, it's that the level of understanding required to achieve excellence ( an A in the majority of US and Canadian schools) keeps dropping so that parents won't complain when their child gets a B or C instead of the A the parent thinks their child deserves. Which means that when held up against other countries where the A margin is hardly ever achieved, they come out below.
I am half in agreement with the OP, I don't agree with teacher led learning really. I do believe that most people excel at topics that they are interested in and choose. I do agree that in the US and, to a lesser extent, Canada the bar for excellence has dropped so low that A in these country is the equivalent of C in other countries. So obsessed with this ideal of "excellence" we are, that the definition of excellence has become "average".