We create our own problems among young people in language usage when we do not tell them that there are different registers of speech and they need to notice and understand what they are already automatically doing. If we do not walk them through this, they notice that their communication with their friends, which they think is the most important thing in the world to them at this point, does not follow the rules that teachers and adults in authority are insisting on, yet seems to work quite well in communicating and getting them what they need socially. This is how they conclude that the conventions are unimportant or even meaningless, and they have to do repair work of learning late a skill they should have been practicing situationally all along. We can save them the trouble. Use what you like with your friends. Speak differently to adults, and be especially careful in any writing assignment. Toggling among them is not a problem. This is particularly important with black students whose informal language has greater variance from formal written English. They can learn the latter quite well, especially if we aren't spending time wrestling with them uselessly over the former. It turns out there is no advantage to dropping "very black" English in favor of "medium black," or even "lightly black" English in peer group exchanges. This is hard for me to hear, because I have made some effort to keep my speech close to formal standard and have felt that was good for fluent writing. Apparently not. All it does is mark me by class and background - which is fine with me, but not the same thing.
Transcripts also reveal that none of us speaks in formal English anyway, nor all that close.
There turns out to be no correlation between children's usage of social media abbreviations like "ur" or lots of emojis and the number of errors they make in formal writing. So also, informal speech uses conventions such as "quotative 'like'*," in which someone says "And then I was like...and she was like..." in which we reference what was being said without committing to it being the exact words. Such things are not random, and the usage rules are often quite subtle and difficult to explain to learners of the language. Yet they are stable. One picks them up only in context from fluent speakers. We destroy our own credibility when we insist on formal communication rules in informal contexts.
Interestingly, Andrea Lunsford and Karen Lunsford have learned that students made about the same amount of mistakes (a little over 2 per hundred words) in the 2000s as they did in 1917 and a few points in between in their college first-year writing assignments. They write essays 2.5x longer now - and they make a different type of mistakes. They used to make spelling errors, now they improperly cite, and in-between there was a tendency toward wrong word choice.
*Linguist Alexandra D'Arcy at the University of Victoria is the expert on the many uses of 'like.'