I need a team building exercise for a group of 20-30 teenagers?

it will be done before swimming so nothing too sweaty... also can be done in a conference room or small area outside... please help...

Here are some ideas for you.

Have them all sit in a large circle facing away from each other. One at a time, they must count up from 1 - 50. The catch is that only one person can speak at a time and the person to the immidiate left or the immediate right of the person who gave the last number cannot say the next number.

This teaches them how to work with each other to accomplish their goal.

Another game is called TENNIS BALLS

Divide the participants up into small groups of about eight to ten people and have them arrange themselves in a circle. Give a tennis ball to one person and explain the rules of the game:

Each group is in competition with the other groups in the room. The group who can complete the most "circuits" in a given time will be the winner. A completed circuit occurs when every person in the group has touched the tennis ball.
Only one person in the group can touch the tennis ball at one time (therefore the ball must be tossed rather than passed.)
If the ball ever touches the floor, then production must stop for one-minute.

Have the teams complete a few circuits to get comfortable and begin creating patterns that make them more efficient. The facilitator may want to stop the groups and get feedback as to how they are becoming more efficient and help them understand that this is a natural progression in business as well. Have the groups continue to complete circuits, but as time progresses, the facilitator will add additional rules to make the process more difficult.

A Co-Worker calls in Sick--Remove one of the group participants and tell the group that the participant called in sick. After they complete a few circuits, remind them that just because someone calls in sick, doesn't mean that that person's work doesn't need to be completed. (They will probably have just continued to complete the circuit just as they had before the person left.) Remind them that each of their last few circuits have had one fewer touches than before, so they do not count. Someone will have to pick up the slack for the absent person. After a new pattern is established, have the person come back.

Double Production--Throw a second ball into the mix and tell the group that our client wants us to double production. Only one ball can be held by any one person at a time. You can add a third or even fourth ball later.

Diversity--New federal legislation states that we need to include more minorities and women in our production line, so every other person who touches the ball must be either a woman or a minority.

Use your imagination to come up with other rules and be sure to have a prize for the winning team. At the end of the game, ask the group how did the game relate to things they face in the business (or sports) world.

Here are some more ideas:
http://www.briandbiro.com/team-building-...
http://www.wilderdom.com/games/descripti...
http://www.wilderdom.com/games/descripti...
http://www.teambuildingportal.com/games/...
http://www.teambuildingportal.com/games/...
http://www.teambuildingportal.com/games/...
http://www.teambuildingportal.com/games/...
http://www.teambuildingportal.com/games/...
http://www.teambuildingportal.com/games/...

I just did this at a camp...put rope in a rectangle shape. Then, depending on hoe many people are in your group, put that number of paper plates. Each plate should have a number, and they can be written any number of ways and you can skip numbers too. Then, have the group work together to find out a way to touch all of the numbers the fastest way. The problem is that only one person can be in the rope by the plates at one time, and the numbers have to be touched in sequence. People will probably think that less people will work better, but if everyone works together, after a few times they should fiqure it out.