I accelerated in Mathematics. I basically asked if I could, and the maths coordinator obliged. There were some logistical problems, but I got by doing about 4 periods a week. If the school is concerned about you missing classes, ask for a compromise (i.e. missing out on some periods of different classes). If you are talented student, they should recognize that you are capable of attending fewer classes than the rest of the school. If they fail to recognize this, remind them!
I did the Year 9 Adv. Mathematics course in the Christmas holidays. I would recommend people skip the Year 8 course AND the Year 9 course if you achieved first in your Year 7 mathematics class and like the subject: those years are very easy if you are keen.
Accelerating in courses is a necessary requirement for eligibility in the HSC Distinction courses (Philosophy, Cosmology, Comparative Literature) which pay enormous dividends.