I enjoy using warm-ups that encourage a sense of creativity and movement ownership while allowing enough structure to hold the more insecure dancers in every group. Rotating follow-the-leader allows everyone the opportunity to be a leader and a follower and gives each person control of the length of time that they lead.
Begin by explaining the activity with volunteers. Gather a couple dancers and remind the group of the game follow-the-leader. This version will be done in a line, rather than the dancers facing the leader and copying his or her movements. Dancers line up behind the leader and the leader travels through space with the other dancers following. Once the leader is ready to stop, the dancer finds stillness, turns and taps the person behind him or her, and then travels to the back of the line to be a follower. The dancer who was tapped is now the new leader. This pattern continues for as long as the instructor would like before moving onto another activity.
Break your group into smaller groups. I enjoy doing this in groups of four or five dancers, but have seen wonderful work in groups as little as three or larger than five.
Remind your dancers that they do not have to perfectly replicate the movement of the leader, but to follow to the best of their ability. Encourage leaders to choose movement that is accessible to all movers. You can add themes playing with tempo, levels, space, or body parts, but I enjoy keeping this exercise as free as possible. Sometimes my younger dancers RUN for most of the exercise, which can be exactly what they need before the rest of their work, but you can always redirect with gentle feedback such as, "Try a new pathway!" or "See if you can take weight into your arms as you lead!"
I encourage you, as the teacher, to join a group as a personal warm-up as well as a simple way to bond with your students.