Defamation is one of the more complex and difficult to prove causes of action out there. They are expensive and take a good deal of time to prosecute. The heart of defamation is showing that there has been economic damages resulting from the publication of the defamatory material. There is something called defamation pe se, but I think that would be very difficult to prove for a child and in trying to prove it an obtain damages, you may cause more harm to the child than good, i.e. you need to engage in a serious cost/benefit analysis. If you did prove your case, if the perpetrators parents are "judgment proof", you would hyve gone through 2 to 3 years of litigation for nothing. You need to discuss this in person with a personal injury attorney downtown to determine what is best for your child
The information is for general information purposes only. Nothing stated above should be taken as legal advice for any individual case or situation.
Around April 2011, there was an Issaquah incident similar to what your daughter is experience. Two girls, 11 and 12, were charged with posting sexual images linked to another girl. If I remember correctly, one girl pled "guilty" and the other girl was found to have committed the offenses. (This occurred in juvenile court; so, the girls got sentences appropriate to juvenile court.)
You can use the search function of the major local newspaper to find the newspaper articles on this incident. Search for "issaquah facebook bullying".
My point is that the prosecutors do take this type of incidents very seriously. Given how easy things can be done electronically and how little thought some children put into their actions, damages can be quick and heavy.
Besides the school, you can contact the local police and prosecutor.