Although often students can have the best teachers and be no better off unless they put in effort themselves, a good teacher can make all the difference. I;ve had first-hand experience of bad teaching in Year 10 and the impact it had on my marks and performance, no matter how hard I tried - I went from a top student to an average student with no confidence, and that was reflected in my marks. I'm still working to recover from it. The issue is that, if you have a bad teacher, you have to devote so much time to teaching yourself what you would normally have known that you miss things, there are things you don't have the resources to learn, and so on. I find bad teaching has a huge impact in English - this teacher I had in Year 10 had us do basically no homework or writing practice the whole year. And those analytical skills are really important. So in that case, I'd blame the teacher, not the student.