I've been a cafe manager at an Elem school for 10 + years. Yes kids r brats, disrespectful, do not listen, but i have found, not for all of them, but most, when u speak and approach them in a more positive way, the outcome is better. Since talking to the parents didn't help, how about talking to the kids and explaining how their behavior is tormenting your dog. If this has already been tried i apologize, then I say break out the water hose and let them have it.