Split the kids in to two equal groups. If you have an odd number, have one kid play the leader. Have one group walk around the other group. When the leader yells freeze, the group walking must each freeze in to a silly pose. The other group must each pick someone to swap with and must do the same exact silly pose. When all the kids have switched, the leader yells "Go" and the students unfreeze, walk around the other group until the leader yells "Freeze" again.
Defeat the Superhero League
One kid is a super villain. Three or four kids pick superheroes they want to be but keep their identify secret from everyone. Each kid superhero bursts in to the villain's secret lab. The superhero kid must act like their hero but can't say their name. The villain must guess who they are to defeat them and get keep them from destroying the secret lab. If the villain doesn't guess in a few minutes, another superhero enters. When all the heroes are present and have a chance to show their powers, then the villain must guess who they are (within a time limit) or be defeated.
Variation: You can have students also practice this in small groups of 4-5 and then perform it for everyone and have the audience guess the heroes instead.
A group of kids (group #1) act out a scene speaking in a weird alien language. They act out a simple activity like playing baseball or cooking. The other group of kids (group #2) act out the same scene and translate what was said in English.
Variation: have kids translate what was said by the aliens as they act
Variation 2: have a kid play an alien who speaks gibberish and have the other kids try to figure out what he is saying in a scene where an alien visits their school or lands in their backyard
This is good for getting a kid to feel comfortable standing in front of a group by themselves. One kid volunteers to be the "famous person." The kid becomes the famous person by acting the same as the person and using their voice or accent but can only answer yes or no. The other students interview the "famous person" and they get yes or no answers until they can guess who it is.
More improv at: http://freedrama.net/improv.html