Since you have been told by the school that the girls were responsible for the behavior, you might consider calling the police to see if this harassment is criminal behavior. Not long ago a child committed suicide due to electronic harassment. The police might take your complaint seriously. I would continue to complain.
In answer to the question about what they should do, they should suspend the children from the camp for violating the policy against bullying, of course. Perhaps a 3 day suspension, or even a 1 day suspension, will get the parents attention. They should have a published policy for how to discipline children for violations of school policy, and the discipline should get progressively more severe, with repeat incidents resulting in suspension or expulsion. Consult the handbook and see what it says the school should do, and then ask the superintendent why they are not following that procedure. Since the school superintendent did nothing, perhaps you could contact your State Representative, and ask for help. You might also contact a civil rights lawyer and ask if there is an legal recourse in suing the school for failing to prevent this bullying. It might be possible to pay a small fee to get a lawyer to write a letter to the school and or the parents of the kids. Sometimes, just a letter on legal letterhead will get people's attention, even if you did not intend to take it any further.
You also might want to get a new cell phone number for your daughter, and advise her to be more selective about who she shares the number with. And, overall, you might want to set some limits on your daughter's texting, now that your family has experienced the dangerous side of texting.
Consider enrolling your daughter in a self defense course like judo or karate. Sometimes, just the knowledge that you could defend yourself is a confidence builder.
Your child might benefit from some counselling or assertiveness training to help her explore new ways to manage her relationships. It is not her fault that this happened, but there is some reason that these bullies chose her, in particular, to pick on. For some reason, she seemed vulnerable to them. There may be some new tactics that she could learn that will make prospective bullies more reluctant to pick on her in the future.
Also, anything that you can do to help your child build a stronger emotional support system, with friends who help her feel safe and loved, can help. Encourage her to participate in activities or join groups until she finds one she is comfortable with, with friends who "click" with her. Bullies tend to pick on victims who seem to have less "backup". If she is surrounded and supported by friends, she will be less of a target.