'Risking your ATAR' probably refers to the fact you're probably dropping from 12 to 10 units. There's a misconception that I've found a lot of teachers seem to spread about how 10 units is 'risky' and they want an additional 2 units to act as collateral. As long as you put in the effort to do well for the remaining 10 there's no reason for your ATAR to be 'at risk'; having only 10 units means you also have a lot of study periods so you'll be able to maximise your efforts across fewer subjects.
I did bio and if you're genuinely not interested in it I would recommend dropping. I put off dropping it so much and I regret not dropping it in the end because I could've used that time for my other subjects when I genuinely did not like bio and crammed for it so bad just before assessments
