I agree this is a difficult task. I would probably develope a list of songs I was considering and present it to my principle and PTO for review. I may even send it home with my contact info for parental review. If a parent asked for a song to be removed, then remove it from the list. I would definately make the list longer than you think you will need for the event. This will give you plenty of choices if some get removed and options for requests during the event.
I overheard the kids at school singing "I kissed a squirrel and I liked it . . . " They had a whole song about the furry lips, etc... I laughed for days. Where is Wierd Al when you need him?
Music is tough. We had a skate night where the DJ put on a VERY graphic song. I was climbing over kids to get to the DJ booth to get it shut down. The DJ said, "well the kids requested it, so obviously they are listening to it other places." That's not the point. (I'm sure the kids laughed for days about talking the DJ into playing that song at skatenight!) I guess my point is - whether it is you or a DJ, there are tough decisions to be made. Every parent will have a different take on what "appropriate" means and many have never listened to any of the music their kids play.
We purchased our own DJ equipment this year and have now DJ'd two events. We use one of the Ipod mixing systems, and I loaded up my personal Ipod with as much kid friendly music as possible. I pretty much just make the decision if a song in appropriate, based on what I hear. Plus I get input from my two children about what songs they like. Right now, if you focus on High School Musical, Camp Rock, the Jonas Brothers, Crazy Frog, the Cha Cha Slide, Cotton Eye'd Joe, YMCA, and pretty much anything they would play on Radio Disney; you'll have the 5/6 and much of the 7/8. It gets tougher for the older kids. Being that I'm not DJ-ing for that level, I haven't had to make the decision on what would be appropriate for that age group...
One thing that I do find difficult is trying to play some of the popular songs. For example, I thought one of those Grease Remix selection of songs would be great, but then had a parent explain to me how during the Grease Ligtning portion they say "cream" in a context that made the entire song inappropriate. Another good example is Katy Perry right now. She's very popular, but one of her songs is 100% innappropriate (I kissed a girl), and even her other popular song says "Witch" but with a "B" and PMS. I still played the song but made sure to turn down the volume during that one line. I do wish they would make a good clean version of the "I kissed a girl" song, maybe "I kissed my dog and I liked it. The taste of his liver dog treat. I kissed my dog just to try it. Hope my mom don't mind it!"
Anyway, sorry about that, went off on a tandem...
Another good option would be to ask around and see if any of the PTO members know a DJ and see if they could provide a list of what they would play.
In the end I think the kids are the best resource though.
We DJ our own dances too, saves us a lot of $$. I have a subscription to Rahpsody and I download songs for free to my I-pod. I'm very involved with my kids school, and I'm the home mom for all kids in the neighborhood, so I listen to what they like and just download the edited or radio version to my I-pod and we are good to go. Of course I would check with your Principal before playing any songs. My Principal lets me skate on having her check the music only because I have been doing this for 6 years and if I don't let my 4 kids listen to it at home I don't play it at school. Our dances only last 2 1/2 hours so we don't need much music, but I have over 500 songs just for the dances only, that way the kids don't hear the same music over and over. As a rule we play about 20 songs in an hours time frame, but we also have the kids do the line dances for the last 20 minutes. They have a blast doing that. We also take requests, the kids pay $1 and fill out a request sheet.
Good luck and hope all works out for you.
One easy way to get lists together is to go to Barnes & Noble or Borders and check out the music selections that are grouped for young listeners. My son just bought his second "Kids Bop" CD. It has 17 songs on it all suitable for his age.