If you were nervous about auditioning then it might get in the way. What about if you get through to the next stage and whatever happens next clashes with your exams? Getting a "yes" at the audition (which isn't always in front of the judges, sometimes you have to see producers first) is only the start - very, very few people make it to the live shows, and even then most of them you'll never hear of again - often this includes the winner!
That being said, it would be an experience. It's up to you. Are you struggling with your A Level subjects or are you finding them quite straightforward? If you need to work I'd say maybe audition next year or something.. but who am I to get in the way of true talent!
A much better way would be to post a video of you singing on here and I will give you my honest opinion. It doesn't just matter if you're a good singer, it's about your personality. And if your dog has just died all the better.