Hmmm,
Black kids in the inner city grow up speaking Ebonics as their first language and it's not until at least a few years later that they are confronted with Standard English in the schools. Once you learn a first language, you don't have to constantly monitor that you are using it correctly. After learning it, the focus shifts more toward expressing ideas rather than on if you used the appropriate tense, subject and verb agreement, etc. Otherwise think how labored conversating would be with each person double checking his grammar.
People who learn Ebonics in the home must at some point be taught Standard English. And they need to be able to practice it. Many people have taken a different language in high school and promptly forgotten it through disuse. The same applies. What good does it do for a student to be instructed in the classroom for a few hours a day but once he goes home, the family doesn't use it, neither do his homeboys in the hood, nor the next-door neighbor, the grocery clerks, etc. Not seeing the value, he promptly forgets about it. Who here can remember the specifics of English grammar from high school?
I agree with you that eventually the kids are hurt by not learning Standard English when they go to work or muct interact outside of their neighborhoods. But the school systems also fail them by not recognizing and showing the differences between the two varieties so the students can learn to switch between the two. They often wait until the window of easy language learning has closed and then it becomes a much more difficult process.
Anyway, it was a coincidence that you mentioned speaking French because studies have turned up anomalies of black students who aced french classes while failing English. Why would that be? Apparently because there are other dynamics involved besides intelligence.
As for the specifics of how Ebonics developed and why it is difficult to eradicate it, and even if it really should be eradicated, I will send those details to you along with references by email so as to keep down the bandwidth. I also am at work and have to work overtime tomorrow bringing a system online.
Take care.