hellll noo people who say that are total liars like ok everyone is different but legit I've seen the opposite happen so much... of course you have the people who are consistent throughout year 11 and 12 but a lot of kids who did well in year 11 get complacent with their marks and start slipping in year 12 since you aren't learning the same things and you definitely have to put a lot more work in.
trust bro i got like straight Bs in year 11 i was the most average student of average students and honestly i was pretty disappointed with my marks like you are right now. i was so confused last year because i had been putting in effort into my subjects but just wasn't getting the results and i realised it was because I wasn't doing the right things such as looking at the syllabus, looking at exemplar work or reading the marking criteria properly. legit last year i would look at my mark, be frustrated with it and then throw it in the trash without even reading my teachers feedback LMAO dont do that. my mediocre marks in yr 11 gave me the adrenaline to persevere and improve in year 12 which resulted in me being really happy and proud with my marks so far. anyways year 12 will go by super quickly so you need to pace yourself and try your hardest to be consistent so you can get the results you want! ! good luck