Are you sure its aggression or playing? GSD's are herders, they play rough, and your dog is a teen age dog right now. She needs to be redirected when she is showing any kind of behavior that is not acceptable by you. How many classes has she had? Does she get any more mental(obedience) training now? How about physical exercise? Shelters are loaded with bigger dogs that aren't adopted out to families with kids for this reason. Its not an acceptable behavior but I'm not sure its aggression. She is testing her limits and you have to set those limits. Last question is, why can't you work with her?
Misty- Samoyed Mix, Tannor- Golden Retriever CGC
Robyn- GSD CGC, TC, Midnite-GSD CGC,TC, Brennan-Golden Retriever CGC, Batman-Husky/Greyhound , Apollo-GSD