Honestly, screw the HSC.
Don't disregard it, but don't let it dominate your personal life. You should be able to do what you want with your personal life, so I'd say go for it, regardless of HSC or school or whatnot. Just do it.
In 5-10 years time, if you don't go for her, you are likely to look back and regret it. I highly doubt that this would happen if you chose her over HSC, but then again I don't know who you are.
Humans are inherently emotional creatures, so if you follow your emotions you are more likely to be more satisfied with the outcome. Given academia is intellectual, and the prospect of higher education and high income is material, it is safe to assume that you would appreciate the emotional satisfaction of pursuing your "love affair" much more than the intellectual and material gratification that comes from academics and careers.
That's my take on it anyway. And either way, if you have mutual feelings for each other, and you are best friends...is refraining from taking your relationship further going to benefit your HSC marks? If anything it will likely create more tension between you both, as you both want each other but deny it to yourselves for academic reasons. Go for what you want and release that emotional tension by following your desires. And being that much closer to another human being may be useful if you want to work on your schoolwork together, as well as an emotional counsellor for the stressful times.
And besides, if you are best friends and have mutual feelings for each other, is deciding to "go out" really going to change your relationship? It might add sex and intimacy to your friendship (which are healthy by the way, I would highly recommend it for someone who wants to do well in school), but otherwise it will be very much the same. Will it distract you? If the feelings are mutual you'll be distracted either way. So go for it.
I think that the sort of people who are willing to give up their relationships and intimacy for school and academic achievements are the sort of people who need relationships and intimacy more.
That's my advice - take it or leave it.
