When I read your post, especially about the panic attacks etc, I felt compelled to reply to you about this. I suffered from this type of feeling several times during my life especially at your age (I am now 48) and I learned to respect the feeling but to take action to prevent pr minimise this quickly before the state of mind controlled me.
I have had a lot of success learning about how to relax and think clearly, to visualise the outcomes of events, so that these outcomes actually come true for me most of the time. Visualise the success you will have in the HSC, the reults of that success etc.
I visited hypnotherapy practitioners twice and learned how to control the fear, panic etc quite well, however now I find this type of material is available on You Tube. I hust searched for "hypnotherepy studying" and listened to some of the first videos and I can get the right feeling from each of them, and can recommend that you spend a little time to see if this helps at all.
With the right state of mind you will easily complete all of your subjects and get a result that is high enough for you to get to the next stage of your life.