I've been to many foreign lands by myself. When I was in High School my French class went to France. I was 17 at the time. I turned 18 while I was there. We had chaperones and we stayed in a group. However, traveling abroad is not the issue here. The issue is whether or not she ran away on her own, or by force.
I agree that's the main question.
However, I'm also interested in the parents' reasoning. The article said this is an annual trip made by students and adults in their community. It seems very improbable that it's a one-time, unexpected fluke for this girl to have been in a nightclub unchaperoned. That is, I expect it's understood by the students and adults involved that the students are pretty much going to run around on their own.
If the families think this is okay, that's their business ... but I don't consider it responsible parenting, even if the girl is a legal adult.