I wonder if it would weigh on your conscience if the same group of kids does the same thing to another cyclist, and causes a major accident, and you hadn't done anything about it? Part of teens learning to act like adults is learning that there are consequences for stupid actions. If this were me, there is no question I would report it, if I thought there was a chance they would get busted. "No harm no foul" sounds like a cop-out to me, but hey, it's your call to make -- but you did invite comments by bringing it up on a public forum.