No, it's not taught as far as I know, but I think it's readily adopted by learners in an attempt to sound more "real" or "authentic" as an English user, because they see and hear it in movies and songs. I can appreciate the intention if you're stuck with stiff grammar rules and pronunciation, always worrying that you might sound like a robot, but sadly I think this attempt to sound like a "real" English speaker actually backfires: the learner ends up sounding like a poor speaker of English. Most of the time I'd need to correct the speaker/writer, because they've misunderstood when and where to use slang forms such as these.

In everyday speech people often pronounce words in a somewhat careless way. There are certain sounds in American English that, in conversation, are not always pronounced as they are written. So, we often say "gonna" and ''wanna" in casual speech. Moreover, there are regional speech patterns in the US. In the South. for instance, many people do not pronounce "r" at the end of a word. However, the only time it is acceptabe to WRITE in such a manner is when the author wishes to illustrate the speech of the character he is writing about in order to give some indication of what the person is like. He would then write the character's words, not in formal English, but just as they are spoken, Nevertheless, with the advent of texting people adopt all sorts of shortcuts in writing. This is especially true of the young. It seems every generation of youth wishes to have an argot (specialized vocabulary) of their own. Many of my generation, taught to observe grammatical rules, do not care for such writing but I think it's here to stay. Unfortunately, those learning English may adopt such usage, thinking it to be standard English.

Actually, it's not taught in schools. But we usually hear native speakers using "gonna", "wanna" etc. on TV, in songs or in everyday conversations. I guess, learners feel closer to the language they're learning when they use slangs. Maybe they think it's cool. Also they might not know it's wrong to use them when writing.

Hi there!! I think this is because of T.V series!!

For my part, I usually won't correct a text with slang in it. Especially if the writer has used texting shortcuts.