Hi, I got 8 A* equivalent grades, a 7 and a 6, so I feel qualified to help here. (five 9s, a Distinction*, two 8s, one 7, one 6)
I did not revise the year before at all. I used the mistakes I naturally made in exams and learned from them. If I was getting low grades I looked up tips and improved with them (went from a 5 to a 9 in English Language over the course of y11!) but other than that I just worked hard in school and relaxed at home.
I would definitely recommend reading through your revision guides and making notes if you need the information to stay in your head. (This applies mostly to your options, humanities and sciences.) For my language, I just practised the speaking as much as I could (honestly, the only truly hard work I put in was this and I'm really proud of my 7 in French because I was afraid I'd scrape a 4). For science, Primrose Kitten and FreeScienceLessons on youtube were my best friends.
Long story short, you CAN get 9s if you start revising in May. Don't panic! I would obviously recommend you keep up with all of your topics, but if you can do that in school then I don't see the necessity of working for hours afterwards.
***If anyone wants some tips and tricks to improve English Language, send me a message. I've got a massive paragraph I wrote during and after my exams that saved my life - I never seemed to get good grades in my language exams even though I had always done well before GCSEs and had good vocab, structure and grammar skills, so I looked up videos and other people's advice to compile a list of what appeals to examiners.