The video has gone from youtube now, but it's easy enough to find elsewhere. The wonder of the internet!
It looks to me like a fairly typical stupid thing that kids do, and have always done. Mix roundabout, scooter and teenagers and it's an obvious thing to try for a laugh. I'd be surprised if this lot were the first to do it.
I'm not suggesting it was sensible, but I hardly think it's indicative of a breakdown of civilised society. Teenagers aren't especially noted for their risk assessment skills after all. So why the hysteria?
Oh, and what makes them chavs?