IMO as harsh as it sounds, but at the same time as true as it is (without attacking anyone here), no matter which school you go to whether it's a good school or a bad school and whether or not you have a good teacher or a bad teacher, there is no excuse to not doing well.
If you do shit in your school exams, it is no one's fault but yours. All a good teacher does is that he/she makes himself/herself more approachable to the student in order to facilitate their needs and wants in the subject but at the end of the day, it's up to your own hard work if you wanna do well and succeed. If you have a shit teacher who doesn't teach and doesn't bother to help, does that mean you automatically fail? Of course not. All it means is that you take it upon yourself to learn the subject but if you prefer learning in a classroom environment then you get into a group study with your friends and you teach each other.
If you want feedback but your teacher's not going to give you feedback is why not go to another teacher for fedback or get feedback from the top students in your grade? (or get feedback from any band 6 student). Ever heard of a tutor who can be a substitute for your bad teacher? Why not use one? A thing that pisses me off is that teachers don't take their job seriously (although you do have to understand teachers have a life of their own such as having kids to take care off) but what pisses me off even more is that students accuse teachers when they do bad in an assessment. I had 2 shit teachers back in year 9 and they just made you rote learn all the content and whenever you asked them for help, they couldn't help you because even though they helped you by explaining things you don't understand, they were literally talking about all this jibberish you couldn't understand. I ended up with shit grades and got pissed at them. Who's fault was it for my bad grades? Me. Could I of had done something in order to get better grades while having a bad teacher that was rendered useless? Yes, I couldv'e read the textbook and studied all the content by myself as well as ask other teachers and ask peers in my cohort.
Suppose you have scenario A where everyone in the class in a band 6 student (out of 24 students) and suppose there is another class with only 6 band 6 students (in another class of 24). Scenario A has a fantastic super dedicated HSC marker scenario B has a shit rookie teacher. Why did scenario A have more band 6s than scenario B? Was this factor based on the teacher, to some extent yes but mainly it was their own hard work and dedication to learning wheras in scemario B there was only 6 band 6s. Could the band 6 percentage rate of had increased? Yes. How? Those students in scenario B couldv'e taken it upon themselves to learn the subject and get feedback and hep from students, peers and other teachers (not to mention there are websites out there which do provide you academic help). If your english teacher is away on a week before an important asessment task is due (because he/she is sick) does that mean that you'll do bad? No it doesn't because you consult others for feedback. IIRC Crobat said in his guide to english advanced that he had a shit english teacher and managed to get a 96 in english advanced in the end
tl;dr At the end of the day, if you get shit marks, don't blame your teacher (even if it's a shit teacher), blame yourself and no one else because no one is responsible for your learning other than you. You either seek out a tutor and/or get help/feedback from other students, peers and teachers. If you wanna do well, stop relying too much on your teacher because your teacher isn't going to be there behind your back 24/7 to look over you during your HSC. I'm not trying to attack anyone here, I'm trying to be honest with you. Having a good teacher is really "by luck" but it doesn't mean you'll automatically do well