It's probably been said and it's probably an over-used cliche (but nonetheless true) but think of the bigger picture. A good performance in the HSC is one of the easiest and most direct stepping stones towards university admission. Also, consider the workforce; who would you hire, someone who bludged throughout school and got mediocre results or someone who worked well for a year and got good results? Working hard in school (regardless of results) shows good work-ethic.
The HSC is about life lessons/skills, creating good habits and realising your vocation, whether you realise it or not. Why not give it a considerable amount of effort and time? Start with the end in sight!
Don't be thrown off by people who say the HSC doesn't matter. It does matter. Try to see education is a privilege, not a right. Don't take it for granted!