setting a study plan is vital, and let people know about it and make them hold you accountable. Set time aside for any extra-curricular activities and leave at least one day at the weekend free for yourself. on your plan, factor in how long it usually takes you to do homework, and any breaks for snacks etc. typically its advised to do 1hr of study for each subject so you might only get one or two in a night. Study smart, not hard. so instead of taking 40mins to write notes, spend time doing past papers etc. Id recommend getting an app like Forest which tracks your study progress but also keeps you off your phone/device for the time set. If you can do so safely, get a study buddy! I have a friend who is also taking chemistry and we do phone calls where we help each other learn and keep each other on track. however, dont do it with someone who is not going to take it seriously if you are commited to studying.
Having "rewards" for getting certain topics is a great idea, ex: if you finish the heat section in science and have done 2 or 3 past questions then you can have your favourite type of dessert. I know many friends had to leave their activities because they couldnt do them and study, so if you have multiple commitments outside school, you may need to decide on one or two to drop.