My boyfriend broke up with me as HSC started because HSC started and I think that was really stupid. So I don't actually know what it's like to do year 12 and be in a relationship, but still, this is what I think: If you're in a well established good relationship where you both care for eachother then you should stay together instead of causing unesecary heartbreak, especially because you probably both understand what the other is going through so it's ok to not talk every day etc., but if the other person insists on the break up then obviously they aren't the right person for you in the long term anyway. If the relationship is unhealthy, well I can't speak from experience, but I can imagine it would definatley get in the way of your studies and maybe it is a good idea to end it. I've definatley seen friends with healthy relationships maintain them through he HSC, the people who had a bit more toxicity involved ended up breaking up.
EDIT: My above mentioned ex also said recently that he regrets leaving me, so that just adds to how stupid it is to break up because of year 12