You can drop to 10 units in Year 12, and Prelims don't affect ATAR, but should you?
Personal preference: I would stick with it if dropping Bio means going to 10 Units. Having more free-time isn't really cutting it, especially for Prelim where time is more abundant. Having 10 units in early Year 12 would be a major risk for you none-the-less.
- If you were to fail one of the other subjects massively after dropping Bio, it affects some school ranking (I believe..) and there's no going back from it. That's why people often stick with their subjects until Task 2 to see what they're performing well in and drop what they're worst in.
- In the rare case of N Determination in 1 subject whilst doing 10 Units, it means no HSC for you.
There are actually advantages of dropping it though, mainly things such as flexibility and focusing on all the other subjects (but you'll need to do it well). But the final decision relies heavily on you, and heck, who knows if you'll actually do well in Bio.