"Right" is in the mind and eye of the beholder. Nudity has always been a part of the world of art and always will be no matter what the age of the model. The operative words here happen to be "for art" and not for pornographic purposes. While I should think it to be very difficult to find and receive parental permission to do so, I find nothing at all amiss with posing nude for the sake of art and most especially not if the artist is using chalks, watercolors, oils or sculpting as a technique. Photography would be my last choice in dealing with children or teens in an "artful" way.