Maybe I'm just lazy, but my approach would be to show some photos that are generally accepted to be very good examples of street photography and ask the viewers whether they like the photos or not and why.
Of course, you should probably have a back up plan in case no one says anything. 🙂
I second very much the essence of this approach. Street photography is extremely dynamic. In a great composition you "freeze" an assortment of people, color, background etc, that will never be in the same place again in true life. Here a major distinction between street photography and let's say, landscape photography (and this too, up to some extent).
So you cannot walk with your friend at the street and teach him/her, because the creation is in your mind, in what you see out there, and how you see it. Street photography is alive in pictures.
Btw, in my personal case, it happens to me the same at every level of Photography and in every level of life too. Only when seeing big manual color enlargements of a local photog I came to "see" what is out there in Macrophotography. Only upon seeing time and again Vogue essays I tasted the high aesthetic value of Fashion photography.
And the same has happened to me with every other Art and craft, like music, sculpture, and ... shtrudel !
You have to taste it, you have to hear it, you have to smell it. Then the second phase takes place, and this one is not upon the teacher but upon the newcomer: To what extent she/he will feel about what is in front of she or he. Not everybody will become a fan of any brand of Art or Craft, just because she/him is presented with a great example. Every one with her/his own sensibilities.
But one thing is for sure - you will never come to like anything you are presented with low class or bad taste examples.
Cheers,
Ruben