I was predicted an E for GCSE maths. But I got an A and the same grade for A level maths. And then I did well at university studying joint physics and mathematics.
I never studied art or did an AS level and I don't know the specifics of your situation, but I'm tempted to say it doesn't sound like any reason to give up. Just try and find ways to do better if you want to get a better grade.
Hey~ I'm currently doing A2 art so i think i can help you out. At AS the highest grade you can achieve is an A, only the A2 units count towards getting an A*. This means that you may get a higher overall grade than expected as the grades have shifted up. If you got an A at A2 you might get a C overall? But i agree with , you should probably check with your teacher~