This is a game to them; they need to understand that the joke is not consensual, and they are overstepping the mark.
If you don't want to immediately go to the police, then a 1st preliminary option is to report to whatsapp and any other platform they are on.....they all need an account and under 18s need parental consent. By reporting, hopefully the parents will be notified. Also report directly to the parents that you know, advising if it all doesn't stop immediately you are taking the screenshots to the police.
I've been subjected to similar behavior by adult women, my (ex) school friends and acquaintances, who were in their 40s at the time. Some of whom are secondary school teachers themselves were slagging off teenage and current photos of me, my clothes, my home, my parenting, my mainstream (loosely practicing) religion, my cognitive abilities, my dead relatives, and they threatened violence against my dog. Nobody liked me....yadda....yadda....yadda.
It was all because certain members of my wider family, some of whom are now deceased were involved in a particular industry many, many years ago, all above board, morally correct & acceptable, legitimate lucrative industry, which these bullies/people have a jealousy of and misplaced anger against.
So their crazy misplaced jealousy and anger got directed at me, and it escalated out of control to the extent that people called to my home harassing me. I advised the harassers that if anyone every came near my home again, harassing me or talking shit about me or any member of my family I would go straight to law enforcement, and I also advised them that I was tempted to go to the police and merely discuss what had happened with the police so that there was a record of my attending, as if I ever needed to report at a later time, there would be a timeline as proof of the ongoing abuse. I also advised that while the jobless abusers didn't have much to lose, the teachers should know better, and I was tempted to report them to the teaching counsel/regulatory body.
As an adult woman this has been very hard for me to digest, it has been very hard on my self-esteem, and there has been so much gossip about it among the school circle. Those idiots are now terrified of me, they have obviously discussed their actions with some sane people and realized the potential consequences.
Show your daughter that you will do anything to protect her and will always be her safety net and support, don't allow this to continue. You need to be very firm!! Good luck.