As far as I'm aware, Biology does not scale as high as other Science subjects like Physics and Chemistry. Considering the nature of Science Extension, I believe it would also scale higher than Biology. However, with that being said, scaling should not be the basis for keeping/dropping a subject. What you should prioritise is your interest in either Biology or Science Extension and your academic/performance potential in each of those subjects, i.e. will you be able to perform better in Science Extension or Biology? Your year 11 studies/results in Biology are a good thing to consider as they can indicate how you may end up performing in year 12 if you were to keep Biology, allowing you to make a more informed decision. If applicable, your study interests beyond the HSC are also something to consider. While both Biology and Science Extension may be beneficial in terms of tertiary studies in a scientific field, Science Extension allows you to gain some exposure to scientific research, which may be a useful addition prior to commencing your studies.
Another factor that you may wish to consider is the unit difference between Biology and Science Extension, with Biology being a 2 unit subject and Science Extension being a 1 unit subject. This means that performing well in Biology overall will have a positive impact on your ATAR. While this is the same in the case of Science Extension, the difference is the extent to which good performance can positively impact your ATAR, which is higher with Biology than Science Extension because of the superior unit value.
I hope this helps!