An Idea for getting students to do group work

Today my university had an open house for highy school students. With the assistance of Gami, Ayu, another student and my uni and a colleague in my apartment, I did a 20 minute mock lesson for a group of 37 and then a group of 29 high school students. Occassionally, I am called upon to teach "demonstration lessons" at junior high schools. When I have tried group work in these classes it has not gone well. Why? The major reason is the students are not used to working in groups. Today, the group work went very well. Why? Because I had four assistants patrolling the class, encouraging the other groups, and providing assistance or instruction when necessary. I realized that if students are not accustomed to doing tasks in groups and reporting their results to the rest of the class, they need a great amount of assistance at the beginning. The key word is "great amount"; each group needs more assistance than I myself can provide. Next time I have a demonstration class at a junior high school, I think that I will ask 4 of my university students to participate as assistants. To learn to ride a bike, I first used training wheels. To learn to do group work, jr. high students will first need training wheels.