It's kind of hard to say, now when they place a big sign over the picture and the talk is sometimes *piiiiiiiiiiip*.
But most of the time when the girls take of the cloths or if they show the underwear (tho sometimes they show it). Or if they play around with them self. Or when Rin is doing adult stuff and so on.
"23-year old school teacher Aoki Daisuke has a huge problem. One of his precocious 3rd Grade students, Kokonoe Rin, has the hots for him! That`s right, the mischievous Rin has proclaimed herself to be Daisuke-sensei`s girlfriend and is now on the warpath to win him over or get him fired...whichever comes first! That`s not the end of Daisuke`s problems, though; he also has to find out what happened to Mimi, a friend of Rin`s who has yet to attend school."