Khendjer just explained the harsh reality of VCE. There are students that work really hard but still don't get a 90+ ATAR and there are those that don't try as hard but do get it. BUT there are also students that work hard but unlike the other working hard students these students also work smart. This sounds cliché but you have to find a study technique that suits you best, don't adopt other students study techniques, instead modify them to suit your situation at home, school etc.
Saying you will put in 100% effort in everything this year and actually doing it are two completely different things. There will be times you will feel like giving up, but just hang in there. I used to get D's and B's in Bio 1/2 and further 1/2 in year 11 but a close friend explained to me my faults in my study technique and pretty much everything I just mentioned above, and lol I got that 90+ ATAR. Just remember anyone can get a 90+ ATAR, its just the way they go about it. I'm sure you can turn those B's into A's.