I heard examples of people which very good school estimates had problems with understaning other people (especially when talked with young people). When they heard words like: "shizzle" or "dizzle", even "gonna", "wanna", "banana" (just joking:))they had problems. In Poland we're teaching English which UK grammar. This is quite funny, you think that you're good.... but you can't understand another person:) One guy talked with boy who wanted to sell a weed, but he couldn't understand him. He asked this boy to talk without slang.
Very often English teachers don't know meaning of some words.... that's ok, but when he don't want to know what those words mean.... is strange. I know that I must learn English which they require, but it's good to know also slang.
In past I thought that words like "wanna" or "ain't" is use by color ppl (without offense by my side). This is gramar by american, right?
Ain't - means ain + not or something? Is so that why this song title "Ain't No Sunshine" is like this?