I understand, from reading this that a lot of people are under a lot of pressure to succeed from their parents, and I hope that for all your sakes that they only want these marks because they know you can achieve them, not because they want something to brag about or are of the misconception that the HSC determines your whole entire life.
I just want everyone here to remember: these marks are about you. You are the only people who are entitled to feel disappointed about your marks... these marks will only ever effect you. No one else.
So if you get your marks and you are happy with them, no one else matters. Who cares if it is not what your parents wanted from you, you did your best and if a mark of 70 is the only mark you can get when you do your absolute best, then I personally think that that mark is very well earned. Be proud of your efforts.