Why treat it like a 'set'? This is not going to be your typical 'audience', is it? My experience with alternative school students has generally been that somewhere along the line these kids missed a serious dose of someone listening to them. They have very effective walls that can easily fend off being 'talked to'.

Is your friend willing to make it a project instead of a concert? The kids could be asked to bring in their own protest music, probably along the lines of POD-type stuff, you listen to bits of it together, find out what makes this applicable to them, and from there slip in your own "Ya know, that's a little something like "X" (your analogies are your own) You can discuss how well historical protest music worked, and learn about how they feel their world is being changed by theirs.

Mick and Mary have the right idea. The only thing I could add would be to not lay the whole greasy mess out on the table for them. Part of the so-called 'street smarts' is essentially self-preservation. People get sized up really quickly. You can be a source of learning, or you can be just some old dude that came to class--all depends on how you bait your hook.

Best of luck,
~J