AS French (Self-teaching) Watch
Self-teaching will be tough but not impossible and a good grade should be achievable. You'll need to invest in some good books and be prepared to spend time regularly on French.
Writing: get to grips with the grammar with Essential French Grammar for the Sixth Form and Beyond by Caroline Tracey: http://www.amazon.co.uk/French-Gramm.../dp/095706120X
Learn your verbs with 501 French Verbs by Christopher Kendris: http://www.amazon.co.uk/501-French-V.../dp/0764179837
Develop your vocabulary with Mot à Mot by Paul Humberstone: http://www.amazon.co.uk/Mot-Fifth-Ed.../dp/1444110004
Choose your exam board and get the text book (together with the workbooks and the audio tapes) written by the board for all the topic-related vocabulary and expressions.
Beyond all that, read and listen to French on a daily basis. I like the Radio France Internationale website for this - lots of choice, the texts are well written and not too difficult and there is a large section for foreigners learning French under the "Apprendre" tab, where there are audio files with questions and transcripts for practice: www.rfi.fr
The hardest thing if you're self-teaching will be the oral as you'll need to find someone to talk to. There might be a French person (or a French-speaking person) in your community; or maybe one of the French teachers at school might be prepared to take you just for an hour a week. Otherwise you might be able to find someone on the TSR website who is prepared to skype with you on a regular basis.
There are loads more resources out there and I'm sure other people will have loads of other recommendations! Bon courage and let us know how you are getting on!
Its not easy being both the teacher and the student, but the advantage is that you know how you learn best, so use that to your advantage. I suppose how to self-teach French depends on whether you view learning French as a life-long commitment to learn the language, or whether (for now at least) you are only concerned with gaining qualifications.
Once you have chosen your exam board it is more than likely that you will be able to find criteria or teaching materials online, and you will most certainly be able to buy to books which will guide you through what needs to be taught under a certain exam board.
Set aside a set amount of time everyday, and stick to it, be clear with your objectives when you are studying, chose a grammar rule, or verb conjugation or set of nouns or adjectives to learn and then make sure you understand by writing practice sentences.
Try putting sticky notes on objects around your home, and make a point of trying to use what you have learnt (from memory alone) throughout the day.
I have a blog which has tips and ideas to help others who are self-teaching, http://jaiperdu.wordpress.com or you could try joining language forum to get support from other self-teachers.
You'll get there I'm sure,