I'm very torn about things like this new testing program in NYC. Atrios is certainly correct that wealthier students in general will do better due to parental/money involvement. This means they get better schools/teachers and will be better educated/prepared for college, meaning they will get into better colleges...
The other side of this is harder to state: no matter the reason that they are more advanced, it is not a good idea to hold back some children to bring others to their level. I think where this goes astray is that a lot of districts are kind of backwards. Advanced children (whether because of their own ability or because mommy and daddy have lots of money) don't need the best teachers and schools. They will learn just fine with below average teachers. The downside, of course, is that if the best teachers are not teaching "their" children "they" will pull their kids out of the schools: either going to the suburbs or moving them to private schools. Advanced children without wealth will be less likely to have as much of a benefit in this case.
This is a nearly impossible problem to get around without lots of money and a less parochial system. Basically, if we could setup a system that the worst schools in the country had the best paid teachers (I'm talking six figure starting salaries) and the best performing had the lowest (say $30k starting) then we could guarantee that the least advantaged students would get the best teachers. It would really require pretty heavy federal intervention in public schools, however, to prevent wealthy suburbs from creating price competitive schools. (Note: there will be private schools in either case, but even private schools would have a hard time matching a minimum teacher salary of $120k plus benefits.)
I'm thinking something like having all teaching jobs on a scale with a few variables: education level (+/- 0.5), experience (+/- 0.5), school rating (+2 to -1). Most schools would exist on a +/- 1 continuum with a few very challenging schools over +1 (so median school would be 0). A value of 0 would correspond to a median teacher salary of, say $47k. +1 would be double that, +2 triple, +3 would be 4x, -1 and below would be half. That way no matter how much education and experience a teacher has, she/he could never do better than the median at the easiest schools.