Well, the funny thing is that the police actually told my work mate to do some street justice if they returned, even though it's their job. My work mate just wouldn't feel safe trying to do her own "street justice" given just how reckless these kids really are. Besides, if all you do is react the way the kids do, then of course it's only going to escalate to into more, possible worse problems.
The police did arrive straight away, but claimed that because they are minors they can't do anything. I call bullshit on that, because if minors can be arrested for drinking on council property (which they were also doing) then they can through the little bastards into a paddy wagon and cart them off to lock up until their parents claim them. This is, assuming their parents care. I know that not all police are bad (I work with a few in the shopping centre I work at) but these officers just didn't seem to care.