I would think it's potentially anxiety and stress related even at that age. I stuttered more around family and friends than in school because I talked way more infront of people I knew without the stress and anxiety so it was free flowing and I didn't care much because all of my family and friends knew I stuttered. School was rough because although people knew I stuttered they didn't know me to be the outgoing person I truly am. So I was a lot quieter and reserved in school. So maybe she's the opposite? Idk. I would suggest having her talk with a therapist? Maybe a speech therapist and a psychiatrist to see how schools affecting her? I personally wish I had a psychiatrist to talk to when I was developing into the young adult I am today. Wouldve helped for those inevitable life experiences we all go through. Also. Don't be afraid to ask her how she's feeling and how she's doing overall. Keeping healthy communication at home is important so she knows how to effectively communicate away from home.