Self teach A level?Watch
I'm self-teaching A Level Japanese at the moment, sitting the AS and A2 exam next summer. It most certainly is possible, as long as you work hard! I learn about 30 new words of vocabulary and go over grammar every day, plus review old vocab: at the end of the day, I spent at least an hour working on my Japanese, and if I'm doing a practice paper that goes up to at least 2 and a half. Ask your school what textbooks/vocab lists they use, then obtain copies ad work through them. Your school should be fine to enter you for the exam, but you'll most likely have to pay the entry fee, which I think is around £80 for AS and A2 together? Not quite sure about that, though.
So, you've got vocab and grammar covered! Great. Now for listening, writing, and speaking. Listening, the best thing to do is just consume media. French TV, movies, music, whatever you can practice listening and translating with - keep on top of it. Writing: try a site like lang-8, where native speakers can correct you. The more you practice, the easier it gets! For speaking, maybe find a French A Level student and offer to practice with them or tutor them in another subject in return for them helping you with French? You'll need to figure out how you're going to prepare for the oral exam - I suspect if your French teachers like you they'll be more than willing to help out