So call legal aid and ask if they can do appointment of guardians. Maybe you don't even need them, but a notarized statement will do - they will tell you. Then talk to the parents. And as has already been suggested, go to a women's shelter to get away and cool off. Your teen is exploiting the imbalance of power between you and your parents. As soon as you are out, she won't have that drama-rich soil to grow her tantrums on, and will calm down.