I went through this exact same situation last year. My boyfriend was in Year 12 and I was in Year 11. So currently I am in Year 12 about to complete my HSC and he is in Uni.
I literally started going out with him about 1 month before he started his HSC, and during the period of holidays before you actually start your exmas we spent quite alot of time with each other. Looking back on it we were both fairly stupid. In the end we broke up. But now we are back together and have been for almost 6 months.
I know that it is hard, but after going through this and heading into almost the smae situation again, and hopefully a situation you and your boyfriend will face next year (not trying to be cruel, just saying I hope ou two are still together this time next year), but through experiance I have to say that the best idea is to limit the amount of time that you spend together. It's not easy getting through the HSC when your partner isn't going through the same thing. Although you may think that you understand what they are oing through, you can't be sure that you are. Me and my boyfriend broke up because we didn't fully think the situation through. We were stupid, but he has been a great support to me through the last month, and I don't think that I would be in the same state of mind that I am at the moment without him there to help me. I am calm and ready to face what is ahead of me, he is one of the things that has helped me keep my sanity over the whole event.
Partners can be either a great help or a great burden during the HSC. Don't quit on each other...
Don't ask him to spend every moment of time with you. But don't stop seeing each other all together. Take some time, and soon, to sit down and talk And decide what you want to do in regards to the amount of time you spend together. Maybe if you only see him after he finishes an exam, unless he has one the next day, etc.
Goodluck. I hope that you stay together!