Definitely jerk off. In art school we had one male model that that got a boner that wouldn't go down. Make sure your clean (everywhere). Be comfortable in each pose. 20 minutes can become an eternity. Practice poses in front of a mirror. Have a good variety of poses, standing, reclining and sitting. Perhaps take some props with you. The students will pick it up if you're nervous. Depending on how much experience they've had with a model chances they'll already be nervous.
Have fun.