Try having your students cut out pictures from magazines and make a surreal collage. Have them use large photographs for the background, and then find a variety of medium and small photographs to integrate into the background.
The emphasis of the lesson is to teach them that surrealism can be defined as realistic objects put together in an unrealistic way, to surprise the viewer.
I do this lesson with beginning art students at the high school level and they love it.
You can use this for a preview lesson.