I went through quite a lot of stress but i got over it around term one...when i figured out the problem...so my stress levels reduced by a huge amount.
i would say work out what are the things that are stressing you.
is it your peers, ur competitors. well yeah at my school this person who was/is a big "show off" tried to show to everyone that they are the smartest. Then i realised that they are stressed as any HSC student, so yeah some people might just want to show themselves as confident, but really they are not.
Are you understanding the work?
Are you studying consistently?
Are you putting your full effort into your work?
If you don't get something ask your teachers.
If not staisfied with what you are doing at school, get tutored.
Be organised, keep a diary with you. Make a list of what you have to do, its very important IMO.
LAST, but not the least, take short breaks everyday, it helps a lot. well i realised this after the HSC finished. I'd say, going out for a walk, playing some sport, or going to movies might help you relax. All my teachers always told me to go out and relax a bit, and guess what i did, just went to play tennis once.
So yeah..work out what really stresses you, it will help you know your strengths and weaknesses.
I hope this helps