One boy (12) is briefly seen frontal nude after other boys have pulled off his clothes and they throw a stone at his face, so his hands go to his face and you see him completely nude. Before that, you see a glimpse of his penis while the boy is wearing his shorts. A number of characters (aged 10-15) are seen naked from the rear.
The season 5, 12th episode "Stefans Geheimnis": When Stefan (Christian Blümel, 14) realizes that he's gay he tries to cut his penis with a razor blade, standing nude in the bathroom (side/rear).
In a scene were a "new" boy is being trained by getting whipped across his back, the male pimp has a flash-back to his own childhood experience. You see a very fat man (frontal nude) slaping the boy to get him to perform oral sex on him.