Oooh, that sounds awesome!
For a runway, I don't think you should do anything too complicated or expensive. Cafeteria tables are a possibility, but make sure that they're stable and they don't have little crevices (like where they might fold) that people might trip or get caught in.
I think that it's great that you're planning on having student work! I think it's fairly simple to make some simple dresses, if you use the right fabrics. If you know of any fashion design programs in your area (high school, college, etc.), ask them!
I'd love to donate work! It'd be probably difficult to send you some stuff, but if it's for a good cause, sure!