The first time you do this activity have the children take turns being the leader while everyone else mirrors them. You can connect the improvisations to a theme you are studying in class or have the children create archetypes using their bodies. Children act out archetypes all the time: the hero or heroine, the nurturing mother, the warrior, the healer, and so on. The children can first pose statue-like as their archetypes, and then move about the room, to music, as that archetype. If, for example, you're teaching a lesson about how life changes as we get older, you could have the class improvise the process of metamorphosis and slowly turn from caterpillars into butterflies. You might use the theme from Richard Strauss' "Also sprach Zarathustra" (from 2001: A Space Odyssey), or try "Blue Rondo a la Turk," by the Dave Brubeck Quartet, if you're looking for a longer piece.
If you are studying about Moses and the Exodus story, consider playing a recording of Paul Robeson or someone else singing the spiritual, "Go Down Moses." In their actions, it is essential that the children show the proper respect when working with a serious story like this. If possible, teach them the song first, and then have them stand in a circle, facing the middle. Play a recording of the song and have them truly listen to the words and the passion of the music. While they do this, instruct the children to pose like Moses or his sister Miriam and stare at an image, pretending that it represents freedom. Then slowly introduce movement, perhaps a simple line dance, or have one child at a time honor the object representing freedom.
Follow each improvisation with a discussion. Talk about the subject at hand, and don't be afraid to discuss the music as well. How did the music help the class tell the story?