Are you still friends with her? If so, then why?
She obviously is not a good friend and it would be best for you to not be around her.
I have assigned seating too (the school was tired of people not talking to other grades and forming cliques and stuff), and I can agree it sucks. Are you allowed to move around the table (or whatever you sit at) to maybe get away from them?
I'm so sorry that she was so mean to you. No one deserves to be treated like that. I hope your situation will get better (it will someday), and I know you can get through it! Good luck on anything you do!
Btw, even though she is not nice to you, try your best to show kindness to her, even though she may not deserve it. Try to love your enemies, and be kind even to people who aren't kind to you!