If you have a friendly relationship with someone, and know they are doing something wrong, and feel morally obligated to report them in order to stop them hurting someone (the example comes to mind of a cheating spouse with an std), I would think the best thing to do is to warn the offender, "I know what you are doing, so do other people, and if you don't rectify this in x time, I will tell on you."
I would only think that applies if you have some kind of friendship ties to the offender. If the offender is not someone you have been friends with, and you intend to report them regardless, I don't see any obligation to issue a warning. And in this case, since many people know and the offender seems not to care, it likely woudn't do any good anyway.
I agree that the swipe about the offenders socioeconomic status was unneccessary, and makes this sound more like a snobbery thing, than about protecting the interests of the children in the district.