Thanks for being interested in the kids and getting involved. Parental involvement is so critical to the lives of children, and you are a great example of parental involvement in action!
I'll answer a few of the questions. First, you'll need to plan a minimum of 2 square feet of space per chicken, assuming that you are using a movable coop (ala Joel Salatin or Andy Lee). A good reference book on the coops is Andy Lee's Chicken Tractor book.
Secondly, the perimeter fence should be high enough to keep out predators. I personally prefer 4 feet tall fences to keep out most dogs. A chain link fence would work well for keeping out most predators and would fit into an urban landsape. You may need to address about smaller predators such as rats or other creatures. Personally I use poultry netting (Premierone fencing on the internet). It is electrified and keeps out everything except hawks. I'm not sure you could use an electric fence around kids without some kind of legal headaches.
Third, rabbits in the rotation... it depends on how you have the rabbits housed. I like to use a chicken tractor design, but staple 2 or 3 inch mesh wire on the bottom to keep them from burrowing out (which they will try to do). Always have the chickens follow the rabbits in the rotation. If you could leave a week or two between the chicken and the rabbit rotation, that will let the rabbit "leavings" attract "goodies" from the soil and environment - the chickens will love you for it. Unfortunately, rabbits don't do well in the rain, so you might want to build a regular rabbit hutch for them and let the chickens rotate under the hutch periodically. Then you can have a more secure enviroment for the rabbits.
Coops on wheels, hmmmm.... the largest movable pen I have is 12 feet by 12 feet, and it takes a lot of strength to move it even with the wheels. You may want to consider something smaller like 4 feet by 12 feet. Easier for the kids to help move.
Just some thougts for your consideration. What happens if the kids grow tired of caring for the animals. Who will take care of them? Also, do you have hawks around the school? I've seen some in even large cities. Hawks are hard to defend against.
Hopefully others will jump in and write more advice, but the Andy Lee book should help get you started. If you have any more questions, feel free to write me a message or just reply here.
Best of luck, and thanks for doing this for the kids.