I'm pretty sure it's illegal to provide liquor to minors....why would another ordinance be needed?
Afraid I kind of have to agree with the police on this one -- there really is no way to know what's in a container. But they should make note of WHO and WHERE for that event and put some patterns together. Next time, they should be able to catch folks in the act.
And it's important to catch them. The last thing you need as a law enforcement agency is for "the word" on the street to be that you'll turn a blind eye for special people or special nights. That would be a setup for a lot of bad things -- like fatalities.
As for those who think it's harmless fun, a right of passage, or "safer to stay at home" are sending quite a different message -- it's OK to break the law, it's OK to disregard authority, and that they are "special"...as in, above the law.
Shame on them for that lesson.