If the girl had made the trip on her own, or her parents trusted her to go on her own and take care of herself, I'd agree with you.
But from the sound of the "chaperone" arrangement, it seems as if someone (parents, college officials, etc.) put an older person in charge to watch over the three girls, and whether the girls were "adults" or not, that person had a responsibility to be, well, responsible for them.
If that's your "job", you should take it seriously and *do* it, not just accept the free trip and then let your charges run wild.
And for all we know the dead girl may have still been living at home with her parents while going to college, and having a chaperone was their requirement for allowing her to go on the trip ("my house, my rules", regardless of how old the offspring is).
Where is the anger at the person/people who killed this girl?
I'm not sure someone did "kill" her. It may have been some sort of accident on her part (drunken or otherwise).