[Grant Barrett]

I get this question a lot and my best advice is to not teach slang, but teach about slang. First define slang for them in a broad way using multiple non-dictionary sources, since the dictionaries are too brief on the subject of slang, except perhaps in the frontmatter, and explanations of what slang is vary greatly. Then choose a couple of slang words that are homographs, homophones, or polysemes of other, regular words. "Cool" is always a good one. Talk about how the slang meanings vary from the Standard English ones, about how words change over time, and how we have different registers of English.

Another way to teach about slang is to have your students keep a slang journal for a few weeks. They would record the slang word, where and when they heard it or read it, what they think it means, what it reminds them of (other words? a place? an older generation? a pasttime?), and why they think it's slang. Then, at the end, you can compile all the journals so that all the students have a copy, and they're usually pretty happy about that.