First, and most important is that the diagnoses are correct and that other possible causes for the problems have not been missed. Has she had formal psycho-educational testing?
First, might I comment that six-year-olds, especially boys, often develop unevenly. Their language and/or motor skills might be more advanced than the other.
I share your concerns. One option is to speak to the counselor who works at the middle school she will be attending. Ask for guidance and for help when she arrives.
When someone has ADHD and also has an anxiety disorder, it is not uncommon for the medication used to treat the ADHD to exacerbate the anxiety disorder.
ADHD is a neurological disorder present at birth. Thus, the history of hyperactivity, inattention, or impulsivity has been noted since at least age seven.
Your brief description suggests that this high school student has been through a lot. He probably does not feel very secure, safe, or good about himself.