Found: Girl sent to teen boot camp/ wilderness camp. Whistleblower of the abusive conditions

I read this book around 10 years ago and its target audience was teens. The protagonist is sent by her father to this camp/ institution to improve her behaviour (I can’t remember what this was). There she makes a group of friends, one is overweight and has been sent to this place as a ‘last resort’ by her family who has sent her to multiple fat camps; this friend, as well as several other overweight girls there are forced on what they call death marches with very little food by the camp councillors and I think she either falls dangerously ill or gets injured. Another friend she makes is a girl who seems very self-possessed who has a choppy haircut that was the result of growing out her hair after she shaved her head. It is later revealed that this friend is actually recently 18 and can leave at any time but has decided to stay because she feels safer in there than free.
One of the councillors is a man and he runs ‘therapy’ sessions where he frequently says his catchphrase (something along the lines of ‘you’re in denial and I don’t mean the river’).
The protagonist manages to escape the grounds and take a file that she and her friends had collated detailing all of the abuses they and others have suffered at the hands of the councillors; she gives the file to a reporter/ journalist who dismisses her as whiny and spoiled. When she leaves she forgets the file and the journalist goes through the file and realises that she was right and he exposes the camp causing it to be shut down and the girls to escape.

It’s set in America though I can’t remember which state. I think the cover may have had fireflies on it but I’m not sure. Any help would be much appreciated!

This is Sisters in Sanity by Gayle Forman.

That’s it- thank you so so much!