You've got to be careful with sixth form as A levels are so important, you obviously know this and it's really great that you're recognising the problem that your friend could potentially make. The fact of the matter is that there isn't very much you can do, if she's stubborn and she can find enough reasons to stay, she will. She can only drag you down if you let her, the best advice I can give you is to maybe distance yourself from her - in a new sixth form you can make new friends, try and find some that are as focused as you are! Is she definitely decided on the same sixth form as yours? If she isn't, maybe try and give her some reasons to go somewhere else?