tough love, that is what it comes down to, have consequences for her behavior, having teens of my own I understand, my oldest just turned 16, it isn't easy even when it is your own. draw up a contract if you have to and get her to sign it and the consequences have to be what they are, don't back down with what you agree on. You wouldn't take that kind of behavior from an adult, why take it from her? there are consequences for everything we do in life, she shouldn't have to skip them just because she is in foster care.