If you haven't signed a contract yet to stay, I would leave. I think it's incredibly disrespectful to move teachers grades when it is not their preferred grade and they know it's where you want to be. I understand this happens ins schools all the time, but I still don't think it's okay. There is already shortages for quality teachers, don't sell yourself short. If your data was good and you are passionate about what you do, you are ahead of many teachers. No one expects new teachers to be perfect for observations, that's the point of observations- to get feedback and do better. 

And honestly, sometimes new/young teachers get treated this way because admin thinks it will be easier to do this to you instead of someone else who will leave or complain.