Hey pal. There may be a theoretical bell curve out there which limits how much certain classes of people can ultimately learn, but today's curriculum makes it totally irrelevant. I have first-hand seen this black school in Houston (http://www.imanischool.org) blow the doors off of the best students that predominantly white schools had to offer in math and spelling - and it is very, very, difficult to cheat in a spelling bee when everyone is given the same list of words to study, and then told to spell them publicly. And I seriously doubt these kids are cheating in math. The difference is that they don't play with this experimental crap at the Imani school - they simply teach the basics.
As to the bell curve still existing. Sure, whites still do better than blacks as a group in standardized testing, that is indisputable. And maybe there's some theoretical group maximum that could be obtained if kids had a half decent education - like 1500 (SAT math & verbal) for Asians, 1400 for whites, and 1300 for blacks, but our education system is so lousy for everyone that this bell curve, if real, doesn't even have a chance to factor in (i.e., blacks and whites, as groups, are struggling to simply reach 1000).
So what is going on - easy. For whites is called Sylvan and Kumon Learning Centers. For Asians, it's usually home-grown, community learning centers. But for blacks, in general, it's nothing. With the exception of schools like Imani, the teaching in this country is done outside of what we think of as "school", and if you don't have the resources, you don't learn. And the bell curve is irrelevant.