I believe TT's comment regarding going ahead in a subject because you enjoy it is an appropiate way to sum up this discussion. In all honesty if individuals wish to go ahead and complete all their course content and achieve a really really BIG headstart, by all means go ahead. But, consequentially, this decision all comes down to one's dedication and interest in a subject. For example if John Smith, chose, physics, maths methods, chemistry, english and studio arts as his subjects, and during the summer holidays he went ahead covering what ever content he wanted too, this decision would only be influenced by his desire to achieve personal success in VCE, this is believe is the reason why people consider working ahead in the first place. It's obivious, why people want to study ahead- because we all have our personal goals which we want to achieve and by studying ahead in a manner that is effective and healthy can help consilidate a good foundation of knowledge, and then when we return to school this knowledge is further cemented.
Moreso, this idea of people burning out and twiggling their thumbs in classes, due to their perception of 'knowing everything' is purely egotistical and is dependent how well one wants score and secondly whats the point of going to school if your not going to pay attention to whats being taught, you may as well stay at home

Lastly, i do believe good students who want to succeed do not do this, rather would utilise class time engraining what they know
and practicing so they can do their best in exams and SACS
This was in response to all comments calling people who study ahead as crazy. :DDDD