although i was never in your position...
Since you're the top in your grade in most subjects, Do you think you can sustain that up till the external exams in your HSC year? If so, you're results would not be affected too much from your cohort's marks (i.e. if you top it internally and externally then your overall mark will be a true indicator of how well you did and not a reflection of your school's marks). I am under the impression from the current HSC system that regardless of your school, you can come first in the state in ANY subject and get a 100 UAI IF you top your school internally and then the state externally.
Resources such as textbooks can be easily purchased, borrowed from libraries or even off the net. Do you think your teachers can help you when you need it most (academics wise)? The competition in selective schools often push you to do well but if you're willing to stick it out and study hard, then there really isn't much to it in a selective school.