Hey Nuclear I thought I would throw my two cents in here.
Accelerated learning approaches are generally accepted as a valid means of educating gifted students and much of the literature indicates that students whom are advanced forward a grade are not significantly disadvantaged socially or academically.
However there are cases, as described by Waxler (2004) wherein accelerated learning approaches do not work.
Students grade advanced face a number of challenges, socially they need to form friendships with an older cohort whom may be more matured physically (and mentally) this is no easy thing to do as ageism is very prevalent across many age groups but I would think that high school students would be even more ageist. Older students may resent a grade advanced student for their gifted reputation and the attention they gain which lays the foundation for bullying etc. and a subsequent pressure to underperform to seem less ‘different’.
Academically, grade advanced students might be presented with situations where they are not easily coming top in their classes; this may be very confronting and detrimental to the confidence of the student.
Grade advanced students also face a number of challenges associated with expectations, they are expected to do well and therefore are put under particular strain to succeed. This may lead to a strong fear of failure which leads to deliberate underperforming.
On this note it is interesting that you felt apathetic towards your course work, perhaps this apathy was the result of the pressure put on you to succeed? Or perhaps it was an attempt to be more accepted socially?
Waxler (2004) details a similar instance where a gifted student develops a sense of apathy towards their studies after being grade advanced.
There are of course more challenges to being grade advanced (gaps in knowledge etc.) but these seem to me the most likely to cause academic problems.
In any case I would carefully assess why you underperformed in your HSC (beyond the surface rationalisation) before progressing further as these problems will most likely follow you into your University years and may even get worse.
Heres a link to the Wexler article:
classroom management, effective teaching strategies, teacher resource