Last year at the after-school art program for middle schoolers, I taught a unit on cartooning and caricatures. If EVER there was a perfect age group for caricaturing, it's middle schoolers. Caricatures are all about "picking out notable elements of a person's physical appearance" then "exaggerating them for grotesque or comic effect". Since middle schoolers do this all the time with their classmates and are told NOT TO, and anti-bullying groups are formed to PREVENT "exaggerating physical appearance for grotesque or comic effect", this is the perfect opportunity to take them off their leashes and let them have at.
I had a variety of photo references for the kids (celebrities, sports figures, politicians), held them in my hands in Pick-a-Card-Any-Card fashion and let them select. The Justin Bieber photo reference resulted in wild pandemonium between two teenaged girls: squealing and tug-of-war and running around the room. I had to put them both in "step" (how this school handles transgressions), and cautioned them with a drily-delivered, "Maybe next time a little less hysteria?"