Hey, Feeshiop!
My first word of advice is that if you're not certain about whether you share the same feelings for her, you should reflect on this before you decide if you want to pursue a relationship. Uncertainty is not the best quality to have going into a relationship so if you don't know if you're ready and willing to commit to all parts of it, the wonderful, the ugly and the powerful, then I would advise against it until you find yourself sure. The possibility of breaking up is definitely there but certainty and eagerness can effectively combat that which brings me to the other side of the argument.
If you are leaning more towards starting it, I'm a more recent example of someone who had a relationship (and is still going strong with said relationship) during my HSC. There are many benefits for having someone there for you during what may feel like the most hectic time ever. I actually found Year 12 my most successful year academically despite many people saying that it would affect my grades and my performance having a boyfriend. My boyfriend was highly supportive of my academic studies and was someone who I could confide in when I needed it most. That kind of emotional support is really valuable to have. Your friends may also offer it in their own way but I think because my boyfriend and I had known each other for already two years prior to that, we knew how to respect each other and when was a good/not good time to say or do certain things. Worrying about the ATAR was something I started to do less because I learnt that no matter how it went, I would still have a successful and happy relationship to keep me going.
I think having a relationship, from personal experience, during Year 12 can be a great experience. You'll learn something one way or another through it. If a break up does happen, I'm sure you'll have friends and family willing to support you through it and to make sure it doesn't affect your studies. I hope this helps and do let us know what you decide to do!
Angelina