OY, Bones... I think it goes back to the fact, that kids (and even their parents) have been conditioned to believe that it's acceptable to dress kids in revealing, sexy ways because that's what is being marketed to them. The target age-group for those looks has gotten younger over time... and when your choice is between something that's only mildly racy and something that would've only been seen in Playboy once upon a time... your own values shift. Personally, that particular photo wouldn't have raised a red flag, for me. I have a daughter covered in tattoos, remember? And they're not all "pretty" flowers & such either.
Then throw in the belief that us counter-culture, feminist-types pushed into the mainstream: that how we look is NOT equal to who we are... and well, this is one of the results. Sure, it's not what was meant -- at that time in our society. We tried to teach our girls to not be ashamed of their bodies - so this is one way they're expressing themselves. In reality, this isn't without it's dangers and the girls also need to learn to be aware that not everyone is going to perceive this so innocently. But that would require active parenting - which is harder to do in a culture with so many alternative value systems. I do think the schools sometimes bring these kinds of conflicts on themselves, too - because of inflexibility, one size fits all and zero tolerance rules that are completely irrational, at times. They over-react too often.
I could argue that society's developed a lot more "hang-ups" over sex in the last 40 years precisely because of the ideas that were mainstreamed... and how those ideas have been filtered into popular media, marketing, and the total landscape-background of society. Reality TV is only the latest, most extreme example. I wouldn't want to be a 12 yr old in this kind of confusing mess!
I read a Dear Annie letter the other day from a 15 yr old, who wanted to break up her perfectly fine relationship with her boyfriend, because she wanted more "drama" in her life. She equated drama with challenge, adventure and excitement... with being engaged in life at the grown-up level. I think it's an honest reflection of just what kid-logic concludes when they assess what they're shown of "grown-up" life, you know? It's a sad comment, in my point of view, that she actually ASPIRED to this, as if it were an accomplishment to be proud of...
... it's things like this that have me reading like a madman lately and a lot more social science...