Why on earth are you worried about contacting her? Why on earth would any of you hide behind the school?
This is a serious issue and you should be telling her mother/father what you have seen straight away. But I would ask them not to reveal to their daughter how they found out, in order to protect your daughter.
Please call them and tell them - this girl needs protection and help.
You should absolutely tell her . Imagine if it was your daughter, I'm sure you would want to know.
If they are on facebook the girl and her mother need to know asap if they are to have any chance of removing them . If the boys and girls are at different schools, it is a little harder to deal with, but if the photos were uploaded by one of the boys without the girl's permission, then it is cyber-bullying and they should be stopped. Therefore the schools (both of them) should be told as much information as you have.
You can ask to have your daughter's name kept out of it.
well done twoteens. Hopefully she has a supportive Mum who will help her get over this and move on in a better direction. You did the right thing and should you start to get flack from anyone (in RL) over this, feel free to PM me x