This question comes up time and time again on here and i'll respond in the same way. I think it's unfair to exclusively blame 'bad' teachers. You are at a stage where you have to develop some initiative yourself. In the end, the results are reflective of your own efforts. Teachers are more facilitators rather than hand-holders at Year 12 where they can guide you and help you succeed but in the end you're the one that has to put in the hard work.
'Bad' is extremely subjective - teachers are human too and as a result, they will not be able to please everybody. Their teaching methods may suit some but not others. This will occur regardless of sector, school etc.
Personally speaking as a teacher, I would be having a chat with your teacher privately in the first instance and stating that you aren't getting what they are teaching and what they can do to help. You will find that most if not all teachers ESPECIALLY at Year 12 are willing to explain things in a different way or at the very least point you in the right direction. Using the 'bad teacher' excuse as a reason for poor performance is a cop out, especially if you have taken no steps to resolve it.