Party for Teens

--Tiki torches (keep away from the party area, perimeter)
-little white lights
-have a mini radio playing (ipod in docking station etc)
-pizza or mini pizzas
-bowls of m&m's
-hot dogs
-burgers
-cookies
-popcorn (can even get those cute popcorn holders @ Target)
-chips n dip
-cake and ice cream of course
-sodas in a galvanzied bucket w/ice or ice chest
-Halloween type bags of chocolate bars
-lots of seating, borrow folding chairs or rent them
-maybe some bean bags (cheap at Wal-mart)
-have all the food out. kids get hungry
-game: put a post-it note on each child's back w/a movie star or singer
(they have to try and guess who they are, they get 2 ?'s per person,
have a couple of prizes)
-have a football on hand they can toss aroudn
-couple of baseballs and mitts, too
-horse shoe toss (you can use mini bean bags)
-prizes could be: iTunes gift card, $5 bill Starbucks gift cards etc
Have fun!

We used to hold a "Farewell to Freedom" party just before school started. Everyone had to wear crazy clothes that didn't match and had holes and were stretched out or stained with grass or paint from summer chores (a great way to use those summer t shirts that are worn and stained). Hair could not be clean and neat. Faces had to be muddy or dirty (we did insist on clean hands for eating and had tubs of hand wipes available). In other words, the exact opposite of what you would have to look like for a school dress code (not indecent of course, we're just talking sloppy here).

Then all the food was junk food. Blue food was our favorite: blue jello cubes, blue kool-aid, blue popcorn, blue gum ... basically everything that would be horrifying to a school dietician who oversees the cafeteria food!

For entertainment, we bought water balloons, cheap squirt guns, shaving cream, bubble gum for bubble blowing contests: again, everything that would not be allowed in school.

The kids had a blast. Of course this was held outside, due to the mess. They had such fun rubbing shaving cream in their hair and making funny hairstyles, and blasting each other with squirt guns, and eating the junk food. We made signs about saying farewell to sleeping late, days at the beach, vacations, and drew sad faces and tear drops.

I don't know if that would be too baby-ish or silly for your daughter, but my kids and their friends enjoyed it.

Don't try to micro manage what they do in the backyard. I would set up maybe the bag game and a volleyball net. then set up some tables with pop and ice and water and some chips and hotdogs and burgers maybe cookies rather than cupcakes as icing will melt. set up a radio or cd player and let them have fun. I did this kind of stuff for my kids. I liked being the koolaid house for my kids.

You prepare the main dish and ask other parents to pitch in, desserts, veggie tray, fruit tray, drinks etc

Our daughter and her friends would do the Guess Who game as they arrived. Used sticky name tags and write the name of a famous person an animal or a fictional character. As they enter the party a parent places one of the name tags on their backs. They then have to go around and ask questions to help them try to guess who they are.

We used to have a table with a puzzle going.

Charades
Trivial pursuit questions to teams. Pictionary with teams.

Maybe have some choices of DVD's..

Simon Says
Tug of War
Pinata, (yes even at this age, they will love it).. fill it with mints, gum, sticker, posit notes, pencils, temporary tattoos..

LOTS of photos!!!!

Oh, we've had as many as 120 teens in our yard!!! I have a simple punch recipe I make they all love... simply sherbet with gingerale poured over it.

I provide a lot of finger foods (boys eat a lot), lot of water and sodas.

We usually have a DJ as well. We keep things low key. The teens want to socialize and have a safe place to do so at their own pace. We make sure they are supervised BUT give them some freedom as well.

Another BIG thing is I hire 2 off duty police officers. This REALLY helps keep things in line and helps other parents know that we are watching these kids. Our rule... you leave the party for any reason... you do not come back. The police are a deterrent for any uninvited guest crashing the party, and anyone trying to sneak in something they should not have.

I know you said 20 kids and that is probably not enough for an officer. We have huge parties so I need the extra eyes of the officers.

Have fun!

I love MamaLucky's suggestions, especially tiki torches (they are so cheap at walmart in the garden section). You could also hang up those paper pendant balls, those are awesome. I would grill or do pizza. Have some various drinks (water, Dr Pepper, coke, mountain dew)
Games: you could do the bean bag toss or you could do some 3 legged race with some obstacles in it lol You could do a water gun fight or water balloon fight
Definitely have an ipod-dock radio/cd player so they can have their music playing way out of reach of any water if you do the water fight lol
Something me and my friends loved as teens were silly string haha, it was fun to silly string each other, not really a game but just fun to do

Keep it simple.
You could grill and make a few side dishes.
My girls did a make your own taco's. It was easy and I could do most of it ahead of time. For appetizers I did chips and salsa and a veggie platter. Yes, most of the veggies disappeared. Have both soda and water available. I also had to have a large bowl of m&m's.

At this age kids like to do their own thing, so don't go crazy trying to plan activites. Make sure there is music available. Have some of your outdoor games (badmitten, boccee, etc) set up in case they want to play. Consider having some squirt guns, beach balls, and sidewalk chalk available.

Have fun!