Hi, we keep our animals AT school so not sure on the movement side of things, although I agree that ARAMs or your local animal health (the advantage of talking to the local branch of course is that if you follow their advice to the letter you'd be hard pushed to be pulled up on anything by them!) will probably be able to arrange the same as you would for a livestock show or similar.
In terms of hand washing etc, you will need to have direct access to warm/hot running water and soap to avoid any HSE issues, and plenty of good signage, and not be in an area where people will be encouraged to eat. If you can get them to give you a pitch right near a toilet block the wI'll be open on the day, and as far away from the icecream Man or whatever as possible, you should be fine! The school might need to check its insurance too, we have a bolt-on policy for ours as it covered small animals but not livestock, though I think that's more because of the pigs than the sheep!
Don't be put off by all this though if you do want to go, a few phone calls will solve the majority of these issues.