I think #8 has answered your question the most accurately. It is almost impossible to teach formal English to students who have had no exposure to proper English, either in in spoken or written form. Everything taught in a classroom is undone by the way English is spoken at home and the way it is written by friends and peers. If students are not exposed to proper English while learning to speak, read and write they are unlikely to ever adopt proper grammar as a habit. While there are certainly appropriate times and places for slang and colloquial English, if this is the only type you are regularly exposed to, you see no reason or purpose or use to formal English.
Science shows that very young (preschool age) children have an easier time learning language than as we grow older. The centers of the brain used in language learning are more active and receptive in small children. If children are not exposed to proper English during this time, learning it later is much more difficult, like learning a second language.
If we want to address this issue, it has to be addressed by families and the community working with educators, not by educators alone.