TBH I had friends like that this time last year (I'm y12 rn), and they had to scramble the Easter before the exams, whereas I started at the start of the Christmas holidays (revising for GCSEs that is), and thus the whole year was wayyyy less stressful as the revision was more spread out. Also, if for some reason you can't make it to an exam (e.g. a severe, sudden health issue), the school can use your mock to show the board what you could have gotten, so it pays to do well in your mocks just in case!