I did not go on a drum corps tour, but I have gone to Europe a few times with different musical organizations. When I went, we would have these square pieces of material and use long, skinny tools to put messages on them. Then we would put it inside a thin case and put a sticky little square in the upper right hand corner. They would magically be taken to our parents. I am not sure how, I think owls might have delivered them.
(All kidding aside, I think cell phones are ok as long as kids don't abuse them. Where I see the most abuse coming in is kids talking to their boyfriend/girlfriend back home and not making friends in the corps.)