Why did you swap from bio to senior science?!
Bio is way better in year 12 than year 11.
Also, it's very easy to get a band 6 in senior science. Don't worry about it.
Just walk in willing to learn.
Dats it.
I was average in year 10 science (came like 30th or something) and in prelim I came 6th/20 just by paying attention most of the time and stuff.