When I lived in Paris, I got my hair cut for free at the Toni & Guy school near the Louvre. They have specific days of the week when they offer cuts and dyes. My friend went with me on a whim, and the instructor asked her to stay to be the model for his afternoon demo. Just call and find out what time to show up. The only downside is that this is artistic, not just technical practice for the students — meaning that you have limited say in how they will cut your hair. But my experiences were very positive, and the instructor hangs quite close to the students to make sure there are no disasters.