I'm sorry to hear this. The people at my 6th form who get As and A*s do work hard and use most/all of their free study periods productively. Perhaps you need to review your study technique. Everyone essentially learns the same content but we have different teachers and revision methods.
I study Physics (one of my A-levels) and I didn't personally feel that the way one of my teachers taught worked for me. I found YouTube videos really helpful and it's good after a lesson to go over what you have just learned to really understand it. I find that if I understand something you either remember it or easily can brush up on the content if it slips out of your mind.
Perhaps your revision technique needs some attention. After a lesson (or during the week), I make Quizlet flashcards on the topic and Physics and Maths Tutor resources are really helpful (not just for Physics and Maths - Chemistry, Biology etc). Once you know and understand the content, you can then practice questions. Any questions you get wrong, make a note of them in an organised way. Make sure you understand the right answer. Put the questions you got wrong on flashcards etc. Then try and regularly go over topics so you keep them fresh in your mind.
In summary, make sure that during a lesson you understand the material. Review the material by watching a YouTube video and making flashcards on the topic/summary notes. Test yourself with the flashcards until you know and understand the content. Practice questions and make a note of questions you got wrong. Review flashcards regularly.
Hope it works out for you. This is just some advice that seems to work for me. Good luck!