One thing that can save you time is if you wash and soda soak all your shirts and then spin them out in the washer ahead of time, then no one has to do any wringing. You can store them in garbage bags or rubbermaid boxes according to size for easy distribution.
If you can get your hands on an old blender, you can blend up 4 cups of dye at a time, and just keep at it until you have your gallons of dye ready!
I like Steve's tyvek tag idea -- i may use that next week with my son's class!
For the ease of choosing designs, you might want to make up a couple sample T-shirts in easy designs, then the kids can choose whether they want to make design A, B or C. Before the whole thing, show your helpers (or a couple willing parents or older kids!) how to tie up the 3 basic designs (or whatever you choose) and then they can go around and help everyone so that you aren't running around trying to help all 30 people at once.
have fun!
nicole
Another idea is to use activated dye so you do not have to soda soak the shirts. Nicole is right about avoiding the wringing of wet soda soaked shirts. I've been there and it's not good!
I use a hand blender with a wisk attachment to mix dyes. It's easier to clean (just turn it on under running water) and I have mixed as much as two gallons at a time with it!
Again, Nicole is right--pick 3 designs and leave it at that! Maybe a spiral, a scrunch, and a bullseye (as much as I dislike the latter personally). They are quick and easy!
Steve