I have the exact same problem. I'm in year 11 now and I couldn't even answer half the questions in a core paper. I used to be good at science, then in year 10 I got my current science teacher, forgot most of the previous information (which is mainly my fault for not revising) and also he failed to teach us additional science properly. I could go on for ages explaining why he's awful. I think the worst thing he's done, except from not teaching us anything, is he gave us coursework a week before the 6 week holidays and expected us to finish it IN A WEEK, we only have 4 lessons a week and we obviously can't take it home. He then said if we haven't done it we have to do it at the end of year 11. Thanks for that sir. I'm still somehow working at a B grade (which is probably down to his inaccurate marking) but I need an A, so just like you, I'm screwed.
I suggest befriending triple science students that already have their a* in biology. Luckily I have a few friends doing triple science so they're going to help me out. I'm also going to revise a lot, read a textbook whenever I'm on a train or something, GCSE POD WILL SAVE YOUR LIFE it has podcasts of each subject and all the topics in it so I listen to those a lot, and PRACTISE PAPERS, even if you can't answer many questions, do it. Then either ask another teacher to mark it or find out the answers yourself and self mark (which is what my teacher usually makes us do out of pure laziness), basically just try and teach yourself the subject, it's a lot easier than it sounds if you start now. I started back in august and I already know p1 c1 and b1, it doesn't sound like much but I seriously know them inside out, I figured if I carry this in by the time exams come I'll be fine. Hopefully this makes you feel better, your post made me feel better as now I know I'm not the only experiencing this disadvantage.
Good luck!