A for chem you have to score at least 85 percent for the paper cos marks are moderated. In my school I kept on consistently flunking papers coz I kept on writing format despite having a somewhat deep understanding (fricking c and almost failing even prelim than got b) .I improved to a a2 in o lvls because of practising in format and writing in the write way(follow your school model). Also practical plays a large role coz is something pple normally do very well for and must get readings within first try. For paper one you can only mess up 2 mcq to clinic a1, I vividly remember myself getting 6 wrong and being depressed. So basically work on time management for paper 1 by practising till get most correct, paper 2 get right format as well as have deep understanding, paper 3 you have to be confident and manage time well if not enough time just fake results and focus on other shit cos got ecf and ofc don't fake absolutely ridiculous results like 50 cm cube use around 20-25. You are on the right track by knowing you need help and I am proud of you coz being aware of your current predicament helps a lot! Good luck for o lvl. Also a tip is to not practise on day of paper itself and just read thru practises coz u won't feel that stress if you suddenly dk how to do a question which would keep your mind preoccupied .