definitely! i got an E at AS in Biology, and a B at A2, I retook one exam this year for biology as well and got an A! as long as you work hard, you should be able to achieve it. I did retake some AS exams whilst doing my A2 exams, but I felt that it helped, particularly with synoptic content. The only problems could be that you might not be able to continue with A2 at your school/college, as some are quite tough on re-enrolement, and also you might struggle to get your teachers to predict you a C at A2 if you got an E at AS