iPhone 5s got stolen?

[Aqua M]

In the beginning of this semester, a new girl sat by me in art. Throughout the past 3 months we became friends and throughout those 3 months she would always tell me how she's going to get an iPhone 5s like me soon.
Last week on Monday, I reached into my bag only to realize my phone isn't in there. Worst day of my life. I went to the school officer and filed a police report. She asked if I had any suspects and I answered yes and gave her the details of the girl who sits next to me in art class.
Thankfully I have find my iphone app installed on my phone, except throughout the last 9 days, the phone has been offline. I've come to the conclusion she knew my pass code to my phone.
As soon as I was beginning to lose hope, she uploaded a picture of her, holding my iPhone 5s on instagram. I screenshotted the picture and sent it to my school officer.
Today the girl came in and starting going off about how she didn't steal my phone and all this to me because the cops called her last night.
Keep in mind I go to the most ghetto school in this world and I'm scared to get in a fight. What did I do wrong? :/ help?