1)It sounds like the therapist you have is NOT trained in treating children with trauma. I would suggest asking for a referral to another provider. Are you getting help from a community mental health agency? Or a private practice, or a hospital clinic? Have you asked this therapist if they have treated children with trauma histories? If not, you NEED to find one. Also behavior change in children takes time, more time when there is trauma involved and behavior is related to trauma. Finding Help | National Child Traumatic Stress Network - Child Trauma Home
2) School: Write a letter requesting an assessment for an Individualized Education Plan (IEP). I don't know what state you live in so I don't know what the exact requirements are for your state, but this is a federal law. Ask for an assessment related to her emotional state due to her abuse history. Make sure you do this in writing. In my state, unless you put it into writing the schools don't have to do anything (and no often they will not tell the parent/guardian this when they request it verbally, the school just wont do it). The IEP in my state makes it harder for schools to suspend/expel students who are dealing with emotional issues.
3) Spanking may be stress relieving for mom, but would not be beneficial for the girl. She has already had her body violated. More physcial punishment will not help her feel better. You may end up being identified as just another person who violates her personal space and hence not trustworthy. We tell children not to hit etc, but then teach them it is okay for adults to hit them when they don't behave. It sends very mixed messages.

If she is in foster care with you, contact her foster care worker and ask for help for finding more resources to help you two.