I've been contracted by my local high school to shoot their spring formal dance. There are several different types of shots involved in this:

1. Senior dinner at the school
2. Processional - students are paired up and I'll have an opportunity to catch them in front of a 'set'
3. Grand March - this is a formal dance where they basically are presented for photos
4. Class Group shots - here where I think the real challenge is - I'll be shooting class groups and the entire school, so sizes will vary from 30 to 200. The dance is in the gym, and it will be fairly dark (probably only 10% of the lights on). I don't think I'll have access to the bleachers... So, what do I do about posing a group like this and how the heck do I light it? I have a studio kit with 3 strobes and softboxes/umbrellas...
5. Candid Dance Shots - I have a Canon with a 580ex2, should I invest in some kind of bracket to move the flash further away from the camera? Any recommendations?

Any other advice or tips for an event like this?

thanks,

Jason