And the school was the only thing that all those children shared. The culture of the school is obviously a factor.

You can support and educate your child about nude images, but if s/he's spending 30 hours a week among people pressuring her/him for nude photos s/he is going to need to do a lot of resisting. (Which, in this case, may have increased the "point value" (if that bit's true) and thus increase the numbers of people trying to get pictures.)

> The police shouldn't be involved unless a charge has been levied by someone (e.g. "this person took pictures without my permission")

Children can't consent to sexual activity. Law makers decided that nude images of children is sexual activity. Thus, children can't consent to making nude images of themselves.

EDIT: Also, Bob may not care that Ann has the image. But if Ann's phone images get hacked and Bob's nude image ends up on the Internet Bob may feel considerable distress. We know that some / many people who were photographed during their child sexual abuse will experience trauma from the existence of those images.