Writing prompts for a poetry class

Hey everybody! A few months ago I started helping a friend teach an evening poetry class for high school students. To get everybody going, the class opens with a ten-fifteen minute writing prompt. Every now and again, there's a bit of a struggle to come up with an exciting writing exercise for everybody. So I'm curious. What are some interesting prompts you guys have had from various writing classes? Or are there any particular exercises you enjoy?