Has a teacher or employer ever criticized you for using slang in your writing? On Twitter and Facebook, I questioned the recent addition of chillax into the Merriam-Webster Dictionary. Chillax? That trendy expression favored by schoolkids and skater boys? Bah! That's slang, I complained. Doesn't belong in a dictionary. Stop the madness. And so on and so forth.
I've climbed off my soapbox and softened my stance a bit after viewing this video, in which an editor at Merriam-Webster makes a great case for why slang is — and must continue to be — included in dictionaries. Every English teacher needs to see this as a reminder to stay humble.
On the other hand, smart students and writers will always consider their audiences. If your teacher has explicitly told you that she does not want to see slang in your essay, then wise up and don't use slang. If you're writing a cover letter to accompany a college or job application, do not use slang. When you're e-mailing or texting your friends, do whatever you want.
From the comments: Akil suggests that dictionaries should wait for a longer period of time before introducing slang words, to be certain the word is still in common usage. What do you think?