Hi,
I study Biology, Chemistry and Maths at the moment, I'm also in Year 12. I'm gonna go out on a limb and say that, if you were to teach yourself the course, it may be doable (to a good standard, as in B or above) within 3-4 months.
2 months is a very, very risky stretch for a 1-year course that you are to be taught by teachers, not to mention end of chapter/topic assessments. You may be able to catch up sometime in Year 13, that is if you manage your time immaculately.
I personally wouldn't go for it; and if I was to, then I'd start now, rather than waiting until the holidays.
Best of luck :j