I don't think that most slang is bad for a language. It helps to make the language more interesting and unique. American and British English are interchangeable as far as comprehension goes, but the slang for each is very different. Knowing the slang is an important aspect of communicating with native speakers in a natural way, and in understanding the culture of the speakers.
I think that ESL and EFL students should be taught the slang after they learn all the fundamentals of the language and are competent in it. Slang is a way that the native speakers make their language shorter and easier to speak. Since the slang depends on knowledge of the native language in the first place, it is important that students of the language be competent enough to understand the formal language. Since pretty much everyone on this forum can at least communicate in written English, I think that teaching slang is OK here.
Student or Learner