What are the policies your other teachers are working by, e.g. are there other teachers who will predict students a grade up? Are there students for your law class that your teacher is predicting a grade up for? Do some research. If it's a blanket policy at your school, there probably won't be much you can do about it. If however this teacher is predicting other students a grade up, or other students in other classes are being predicted a grade up, I would say go above this teacher as that's just not fair.
And people are being very harsh on you - an A in one exam and a C that was one mark off a B is easily brought up to an A overall. There are people on here claiming they got Cs at AS and brought it up to A*s and As and other idiocy. Ultimately if your teacher predicts you an A, and you only get a B and miss your offer, YOU will be the one suffering because clearing etc is a very stressful process. And if you do prove her/him wrong and get an A, why should you have to do adjustment and probably not have halls etc when your grades are perfectly reasonable to ask for an A grade prediction. I wouldn't give up just yet.