Simply put by most people already, Spanish, French, Mandarin, Japanese, German, English (ESL)... these are all languages spoken by a wide variety of people all over the world, they are taught in elementary school, highschool and university alike because they are languages that are possibly useful in economics.
In Canada I know for sure if you want to work for the government in the upper status jobs you must be bilingual (English and French), and furthermore if you can speak both languages you are almost guaranteed a decent job with the government and what not.
But what I am getting at is, schools teach these languages because they have the potential to be useful.
Ebonics on the other hand is not even a language it's slang, at most I would call it a dialect that originated in the USA and dialects usually only pertain to their specific area. Anyone who is not from the USA and speaks in ebonics is probably someone who just adopted that way of talking because they think it makes them look badass or something, and no offence but people where I live who talk like that just come off as pathetic wannabes. It's a culturally specific linguistic utterance. And anyone who speaks it that's not from that culture sounds some what ridiculous, it would be like someone who is not British to just up and start speaking with a British accent all the time. (That's just an example I know an accent and a dialect, language, whatever are totally different things.)
Therefore it would be irreverent to teach Ebonics in schools, especially since the majority of people who speak in Ebonics speak and understand "Formal English" it would be pointless to teach it in schools. Unless your making a living in a US community populated heavily by Ebonic speaking individuals where would you ever use it? Don't get me wrong, I don't think it's a retarded abuse of the English language like people have claimed in the past (not on here) it's a way of speaking that pertains to a specific culture and if they feel so strongly about preserving it they can have cultural classes after school. In Ontario (Canada) most schools offer after school cultural specific classes for kids who want to keep their heritage, language and their culture.