Gotta protect the kiddies from themselves after all.
L
And of course freedom of association has long since gone out the window as well.
If there was probable cause; e.g., excessive noice, breaking glass, etc., I think the police officers were well within their right to investigate, especially if they were called to this location based on reports from neighbors (I'm making an assumption here).
Of course, if no breathalyzer tests were given, or no pictures taken of the suspects with alcohol in hand, there would seem to be no evidence directly tied to an individual.
I think many of us have been in the situation as these kids. You may not particularly like it at the time, but they were breaking the law, and should pay the consequences if they get caught.
If it is an open party, it's an open party. That's why college frat parties have lists now. No undercover cops can attend.