Can secondary schools do this?

I mean, we had an assembily friday because we're going into last year. Anyway, they were saying 'If you're target is an A* and you're only getting A's you will have to retake you're exams. I'm meant to be getting A* when i can only get A's and B's. I'm not capable of getting a A*. I can't physically do it. I just can't. I work flat out anyway, i need me time too. I know it's important. But i don't want to retake those exams again if i'm happy with that grade. Do i have to? Should i write in if it's that case?