Hm. They might be practicing for something bigger. A summer camp hosted by Hogwarts? I don't know what the reality of the situation was, but it sounds very like "our security's not as tight as normal, but we still take it seriously"; and at the same time, they probably didn't expect teenagers fighting among themselves as something that would get investigated after the fact. It's a perfect setup for a dry run.

Ha, even the place sounds perfect! Small enough to locate the target, but touristy enough to blend in.