The fact that you say your parents would kill you seems to show that you have not quite sat with them to talk about your situation. I'm sure your parents want the best for you but wanting the best for you doesn't mean that they can control every aspect of your life. If they don't seem to understand your situation than it's about time that you start coming out of your shell. Go to the head teacher and just drop the damn thing. The subjects, marks and HSC all come down to you and your life in the future (not your parents or family). It seems as though your parents are trying to fit in to the broader part of the family as your cousins seem to have done really well and they want you to do the same thing. This is wrong that your parents want to showcase you like that, you are not a trophy that they can show off to, you are their son.
As for studying I suggest that the first thing you need to do is change settings. It's mainly a specific place in which we tend to procrastinate (for me, it's my bedroom as it has the computer) so I suggest the first thing you need to do is get away from that specific room. Because even though you remove the items that distract you, you tend to still get distracted as your brain sort of relates the room you tend to study in as the room to procrastinate. When you find a suitable place to study, try to leave all distractions in a different area (I'm sure you wouldn't need your phone/computer to study). As maths is your weakest I suggest putting that as the major priority in your studies. I suggest about 2-3 hours on your maths with different topics in between every 30-60 minutes. But it really comes down to you, what do you think is best for you in order to study? Sometimes methods don't work as it just doesn't suit, listen to your heart, and try to think deeply to the roots of your procrastination. I hope I helped
