I think, if you need it, you should put more focus on catching up and actually studying what you've already learnt at school, rather than summarising a whole years worth of work. To get ahead is a good idea, but perhaps only a topic or part of a topic, rather than the whole course.
Personally, I think reading all your English texts is a good idea, and any texts you may have in other subjects (for example, history extn.). It is also a good time to practice maths (if you do it), but I highly recommend you only practice what you've learnt at school because it is highly likely that you will only waste time trying to learn from a textbook that which must be taught by a teacher.
I say give yourself till Christmas/New Year to begin your school work. Have a break, you deserve it.
For content based subjects, I know I said not to learn the whole course, but perhaps just read through the textbook of the topics you are doing. This is only if you want an general idea of what you'll be learning. Besides, history students, if you actually enjoy history, you will probably take pleasure in reading through the topics!
Obviously the break will be a great time to work on major works. I suggest if you have any you really utilise this time! Don't waste your time "getting ahead" in other subjects if you can use this time to actually make progress on your work. You will never have as much time again to concentrate purely on your work.
Draw up a set of goals or a timetable for when you can fit in your school work. This way you know you are getting everything done that you need to get done. Make sure you put the most "urgent" things at the beginning of the timetable so that you get the most important things done first and so you dont waste time getting ahead in one subject when you need to catch up in another.
I know that I had massive plans for the summer break that I hardly lived up to. This only left me with a sense of deflation and I felt like I had achieved nothing. Don't set your goals too high, but do have them. And don't forget to enjoy yourself, the next 10 months of your schooling life will be quite hectic!