I am tending to lean the way you are--the chaperones are libel for those kids from the time they left Alabama until they return to their PARENTS in Alabama...
Even if she had disappeared after landing back in US..the chaperones should be responsible...
If they didn't want that kind of responsiblity, they should have not volunteered...
From what I have read, they won't be held liable for not protecting the graduates because everyone knew they were not going in order to protect them. They were going to act as little helpers in case any of the kids wanted or asked for their help with anything.