As far as I'm aware, the marks that you receive for your assessment tasks/exams in year 11 are not modified/adjusted to accommodate a low class average, rather they are left unchanged, i.e. your mark will likely be recorded as 20/30 or 66%. With that being said, you shouldn't have anything to worry about at this particular point in time given the fact that none of your results in your year 11 assessment tasks/exams contributes to your HSC results or your ATAR.