"Cool" is positively prehistoric by slang standards. It originated during the jazz culture of the late 1930s, but every generation since has embraced it as its own.It'd be very difficult to teach the slang that is being used right now because it's mutating all the time. By the time you'd researched it and pinned it down, it'd be different.
The inherent attraction of slang is in each generation's opportunity to shape its own lexicon. Why deny that opportunity to nonnative students wanting to be a part of something they recognise as their foreign peers? One should at least try to find a way to accommodate such students.
As I said, I think that most teachers' resistance to helping students learn slang is based on the restrictions that teacher may be under from higher-up, the teachers own language preferences or feelings of inadequacy, and/or the fear that one may be teaching a degraded form of language.