If you're stuck indoors, maybe rent or borrow a couple more flash units and umbrellas. I shot large groups for my kids' school last year and used two silver umbrellas each with four Yongnuo 460ii about 15-20 feet back. Google "photograph large groups" and you will get a bunch of ideas.
Maybe the two 580's direct with no diffusion will give enough light. With your 5DII, you can afford to up the ISO a bit. If you're new to shooting large groups, like I was (and still am) the biggest thing that impressed me from my shoot was the real affect of the inverse square law with light (see the Adorama video below). With a few rows of people, you need to move the light back a bit to make sure the lighting on all three rows is relatively even. If the light is too close, the front row will likely be much brighter than the back rows. And when you move the light back for large groups, you lose a lot of light in a hurry (especially if the walls and ceilings aren't white). Check out the video and the others on Adorama about inverse square law.http://www.adorama.com/alc/article/13036
Another tip, bring your laptop and shoot tethered with DPP and the EOS Utility. It is a big help to see the affects right away and be able to experiment with lighting. Of course, this helps with patient subjects. Maybe ask them to practice singing while you get the lighting right.
Of course, if you can find something outside, then ... nevermind.
Good luck and please update us on how it went.