I go to AT lessons but i would like to get an answer to this question before my next meeting :
when i sit i usually lean back on the back support when i sit like i should with the AT i get a pain feeling in my sitting bones - i feel like they are sitting on the chair and there is no muscle under them like just the bone and it hurts
This is unusual but not unheard of. I assume you or your teacher are using a wooden chair? Have you tried using an upholstered seat? Or a folded towel as padding? It might be worthwhile checking at what point in going from your ordinary way of sitting, leaning against the back of the chair, to unsupported 'Alexander' sitting, the pain begins. Is it as soon as the sitting bones begin to have pressure put on them, or is it only when they experience your full weight?
I would recommend you talk to your teacher about this. You definitely don't want to be in pain during your lesson. Pain can be a major distraction to any learning experience. It could be 'referred' pain, of course, but either way, it's not ideal.
My advice is to talk with your teacher, and experiment, either with them or on your own. There will be a critical moment when you're moving from painless sitting to the pain you're experiencing during which it might become obvious what is going on that you need to avoid.