Hi Christine. It does sounds like she might be having problem in school. You are on the right track. I hope you get to the bottom of the problem soon.
I have spoke to her pediatrician and school counselor and put a call into a therapist. My gut is that it is not sexual, but bullying. The time line in which this started fits.
You should have your daughter evaluated by a mental health professional or the school counselor. It sounds like there is a lot more going on. The professional would be able to sort this out and rule out whether your daughter was or is being sexually abused as the behaviors you identify could be symptoms of sex abuse.