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I've done this kind of thing a few times. The suggestion of a ladder, preferably a 14 footer, is right one. In addition, if at all possible, you want to use the longest lens you can. The ladder will help enormously, but the longer lens you can use the less you'll have fat heads in the front row and pin heads in the back row.
Two umbrellas or similar will work. Three would be better. You MUST check your lights with meter to make sure illumination is the same top to bottom and side to side. That will cut down on a ton of post production work, but still shoot raw. Position the lights as close to the group as possible where they aren't in your camera's view, but even illumination is more important than power.
I'd also see about a lens that will result in better quality than that kit lens. I use medium format for groups, but if you aren't printing big, dlsr will work ok, but try to upgrade that lens. Use some kind of support, even on that ladder. I've used a monopod or a leg or three of a tripod.
Be in shouting voice to the band director. Just tell him to move people you've identified as being blocked or can see clearly or have their instruments in front of their face. I try to give the tuba guys a warning so they can lift it up to get it off their face. Marching bands usually are like military organizations and take direction very well.
I'd use Pocket Wizards to trip the strobes, but if you have two studio strobes and do have them in front of you, you might get away with your on-camera flash hitting their optical slave. Better test that.
You also need to know (not guess) at your depth of field and if you have to bump up the ISO, that's what you have to do and deal with getting rid of noise and sharpening in post production.
If that background is dark and their uniforms are dark or you have dark complexsion kids, that's another factor. Again, helps to shoot raw and will help even more if you can use a tripod.