I have no trouble with not using slang when teaching.
You may have more trouble than you think. One often uses slang without
being aware of it. A real eye opener is to take several years of
instruction in a foreign language and then try to read a work written in
colloquial variants of that language or try to listen to it.
I think Americans in particular are not very aware as to when they are using
slang and when they are not. (I know I have some difficulty with this, and
my conciousness has been quite raised with regard to this through three
relationships with people who are non-native english speakers, not to
mention the 80% of our graduate students who are non-native english
speakers.)
I actually recommend highly taking foreign language courses, in particular,
if you have a large number of students for whom English is a second
language.