Which exam board has been offered in your school? How confident are you in composition/performance/music theory? How many subject do you take/have you done for A2?
It is definitely possible to do a full A-level with A/A* in one year, just keep in mind that you will have double work load than other students-you need to take two: AS Music and A2 Music. I only recommend this to you if you want to study music as your major in university/conservatoire, but probably it is not your intention as you haven't started it earlier.
Music is recognised as one of the most difficult subject in Cambridge, as it examines 3 different areas: composition, performance, and music theory. Be prepared to spend 3-6 hours every day on music if you want a full A-level with A/A* in one year.