Forbidden Letters
Begin the game by coming up with a letter of the alphabet. Try to avoid picking vowels unless you want to really challenge the players. Gather up all of the party guests, and tell them that they cannot say any words that contain that prohibited letter. Assign one player the role of the "questioner" and have him ask people questions in order to see who slips up and uses the forbidden letter. The questioner himself also has to abide by this rule. If he falters, he gets disqualified and replaced by someone else in the group. The last person remaining wins the game.
First Impressions
Prepare for this icebreaker game by gathering some index cards and writing utensils. As your party guests arrive, tape to each of their backs an index card and give each of them a pen. Begin the game by telling the guests to mingle and start conversations with each other. Whenever a person starts talking to someone, he has to write on that person's back his first impression of the person. For example, you can write "Great smile" or "Funny laugh." Tell the guests to keep their comments positive so that no one gets her feelings hurt. Once everyone has had a chance to talk, tell them all to gather around in a circle and pull the index cards off their backs to read them aloud. The teens can all have a laugh and get to know each other in the process.
T-Shirt Relay
Give your teen guests some exercise by first dividing the players into two teams. Line each team up side by side, and give the first player in each group an over-sized T-shirt. Each first player has to put the T-shirt on to begin. Each person in the team has to grab hold of the hands of the person next to her. The object of the game requires the players in each team to get the T-shirt on everyone's body from the first player in line down to the last without breaking the chain of hands. The first team to finish this task wins the relay race.