VERY POSSIBLE. My end of year 12 grades were A in maths D in bio E in Chemistry. The MOST important thing is to be unfazed, dont panic and back urself. Wallah I wasnt that bothered because I knew i could fix up. Here are the steps I took.
1. Literally make a timetable, my aim was to do 1 bio topic a day. If you do AQA bio for instance in the cgp book the topics divided like 1A 1B 2A 2B 2C. It went upto 4 so roughly 9/10 topics which meant I could revise all of AS bio in 10 days. I did the same for chem. And I didnt study timewise I just set out a goal if today I was meant to do all of a bio topic I will do it regardless of it being 30 minutes or 3 hours. I took breaks but the work was always completed. The way I completed a topic was read the revision guide then do topic papers w the mark scheme. Answer the q in my head and the answers I didnt know I would memorise the mark scheme. I never really made notes just not my thing. And yeh within 10-14 days you should have completed all of AS Chem and Bio and just repeat the process it gets quicker because eventually you don't need to learn the content you just jump straight to questions, or you run out of questions (in this case do similar questions from other exam boards) or the topic is genuinely easy now for instance topic 1 in bio.
2. For ucat genuinely once you start revision you'll see putting more hours into ucat doesnt equal a better score. You kind of peak after a bit. i Would say set aside 4-5 weeks for the ucat and roughly 2-3 maybe 4 hours a day. The first week or so you want to learn tips and tricks, watch videos read ucat guides, learn keyboard shortcuts and also do the questions. After that you should be kitted up and the next 3/4 weeks should be questions AND mocks. Aim for a a high score of 3000+ And also everyone has a weak section. Your goal should be to find 2 sections that you can do amazing in, 1 in which you can do good and the other okay/average.
idk y ur teachers said that they're pessimistic people you got this, anything is possible and I don't say that lightly. If you do need any help you can pm or reply here.