When you attent a school, you represent that school even when not in class. Same goes for your job, your community, country or whatever. Now, I don't think your kid should have been suspended, but I think I can see where the school was comming from. If this school is close enough to this hospital for kids to walk to it and do stuff, then the hospital will be compaining to the school if the kids are acting up. The hospital will see the school as being in charge of what these kids do, even though they can't control it. The school has to put a foot down to prevent any other kids from going over there and causing trouble. Hopefully it's just a short suspension and there'll be no negetive effect from it.

Best of luck to you and your kid. Hope it all works out for the best.