I am in Y13 but I got 7888889999 in my GCSEs in 2020 so I’ll try and help you out a bit by subject?
1) Biology - I don’t think you need help here! Nor could I offer it since I got an 8 in my Y11 Biology mock (and real thing since teachers based it on my mock)
2) Chemistry - this was a subject I really struggled with at first. I would really recommend trying to watch ‘Free Science Lessons’ on YouTube. He has the most annoying voice but he’s super helpful and covers the spec very well, as well as including really great examples not seen in the textbook! I would just make some notes from his vids and do some practice papers . I got an 8+ in my mock doing this and sadly because of teacher assessed grades was given an 8 but still good
3) Physics - exactly the same as Chemistry! Also learn the equations - I know you guys are getting an extended equation sheet (that my year didn’t have) but you need to understand what these symbols mean and how to use them in order to answer questions. A massive tip from me would be that if there’s a 6 mark question with equations, you probably need to work with more than one equation - possibly up to 3! I’d try and work through these as once you get it it’s worth so many marks.
4) For Maths, I’d go through the textbook and do practice questions. If you struggle with any practice questions, just ask your teacher for help and that should be useful for moving forwards.
5) Geography is just a memory game! Once you’ve got the essay technique (which if you haven’t, I’d recommend practicing some essays), you just need to remember info - I would do some notes and flashcards. This got me a 9 in geography!
6) I didn’t do music but I’m currently doing grade 5 music theory and grade 6 practical so I’ll let you know what I learnt from the theory in terms of revision techniques (may not be helpful tho, sorry). Practice questions was a big one, as was flashcards for remembering those musical terms and for remembering your keys and scales
7) Loved English Literature at GCSE! I got a 9 and honestly I owe it all to “Mr Salles Teaches English” YouTube channel. I watched all his videos called ‘all the themes of…eg. Macbeth’ and made posters on it and it was sooo useful. I watched his essay technique videos too and did practice essays for my teacher to mark. Flashcards for quotes are great too - have the quote on one side and analysis on the other.
8) For English Language, I would make flashcards identifying techniques (like metaphor etc.) and their definitions so that you remember which is which. I would also look at learning the essay technique - which you can find on Mr Salles’ YT as well (defo recommend!) the final thing would then be to do practice essays and get your teacher to mark them.
Btw I got my 7 in French so all these subjects above I’m offering advice for what got me an 8/9! Good luck