As someone who has gone through something similar to your daughter (although it started when I was 13), I can understand her behaviour.
Your daughter is having trouble processing what happened and trying to make sense of it. I was in denial for a long time about what happened. Only in the past couple of years o have realised that it was abuse and was wrong.
I have PTSD and it is an ongoing battle. But what really helped me was counselling (CBT) and, for a short while, going on Sertraline.
While the abuse was happening, I was a very angry person. The only thing that kept me sane was my music.
Your daughter does need to see a professional. But it is such a hard step. I would try to have a conversation around what she is scared they are going to say. Speak to your GP on your own about your daughter or contact an organisation like MIND. Ask for advice and what would likely happen if your daughter went to have an appointment. Talk this through with your daughter.
Part of PTSD is withdrawal. When I had a relapse last year, I could not go into work. There were only a couple of places I felt safe going to and only a few people I felt safe with. I could not be around large groups of people. It scared me so much. I had irrational thoughts about what could happen to me. My guess is your daughter doesn't feel safe going to school - but may not realise this.
With the counselling we talked through how I reacted to various situations - and it was fascinating and such a relief to work out why I reacted various ways. We figured out the emotions I was actually experiencing. And only then, I could change my behaviour.
It's taken a lot of work, but I am a totally different person.
Unfortunately, it will be hard until your daughter is ready to take the first step to get help. You can't force her. That only makes it worse. So patience is required.
I hope this helps.